HIGH-MAST TOWER FOUNDATIONHigh-Mast Tower (HMT) foundations have been traditionally designed and...

HIGH-MAST TOWER FOUNDATION

Chungwook Sim1, Ph.D.

Principal Investigator

Chung Rak Song1, Ph.D.

Co-Principal Investigator

Brandon Kreiling2, P.E.


and

Jay Puckett2, Ph.D., P.E.


in

Department of Civil and Environmental Engineering1

Durham School of Architectural Engineering and Construction2

at

University of Nebraska-Lincoln

Sponsored By

Nebraska Department of Transportation and U.S. Department of

Transportation Federal Highway Administration

December 31, 2020

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TECHNICAL REPORT DOCUMENTATION PAGE

1. Report No.

SPR-P1(20)

M111

2. Government Accession No. 3. Recipient’s Catalog No.

4. Title and Subtitle

High-Mast Tower Foundation 5. Report Date

December 31, 2020

6. Performing Organization Code

7. Author(s)

Chungwook Sim, Chung Rak Song, Brandon Kreiling, and Jay Puckett

8. Performing Organization Report No.

9. Performing Organization Name and Address

University of Nebraska-Lincoln

Department of Civil and Environmental Engineering

Durham School of Architectural Engineering and Construction

1110 South 67th St., Omaha, NE 68182-0178

10. Work Unit No.

11. Contract

Project ID: 48952

12. Sponsoring Agency Name and Address

Nebraska Department of Transportation

Research Section

1400 Hwy 2

Lincoln, NE 68502

13. Type of Report and Period Covered

Final Report

7/1/2019 to 12/31/2020

14. Sponsoring Agency Code

15. Supplementary Notes

16. Abstract

High-Mast Tower (HMT) foundations have been traditionally designed and constructed using cast-in-place foundation with anchor bolts that are used to secure the tower to the foundation. This type of design requires a base plate that is welded to the tower shaft.

The Nebraska Department of Transportation (NDOT) has recently experienced issues with stresses that this type of design presents

at the anchor bolt/foundation or base plate/tower shaft interface. This issue in worst cases may lead to a premature failure due to

high-cycle fatigue as one of the towers at Milford, Nebraska that fell down during a winter snow storm event in 2018. This research

project objective was to develop an alternative design for HMT foundations with direct embedment of HMT which can eliminate

fatigue-prone details associated with the pole-to-base plate connection which is the primary location of failure. First, the literature

that includes research from academia and industry, current and proposed state of practice from industry, examples of design

specifications and guidelines, and corrosion for buried structures were reviewed. Secondly, structural loads for the typical 120- and

140-ft HMTs constructed in Nebraska and the soil resistance for them were calculated. The structural loads were computed using

the AASHTO LRFD Specifications for Structural Supports for Highway Signs, Luminaires, and Traffic Signals, with a spreadsheet

based on the fundamental principles of structural analysis. The geotechnical foundation resistance calculations were made to check

the vertical and horizontal soil capacity for the typical HMTs used in Nebraska. In addition, further parametric study was conducted

using two numerical software: LPILE and COMSOL for varying soil conditions and foundation systems with different embedment

length and backfill diameter for the service level base moment and shear. Required embedment length and backfill diameter are

provided as a matrix using the LPILE analysis results. Finally, based on the site considerations and constructability, a draft design

and construction specification for soil parameters that can be used for Nebraska soil conditions are provided.

17. Key Words

high-mast tower; steel pole direct embedment; direct embedment

foundation; backfill; soil-structure interaction

18. Distribution Statement

No restrictions. This document is available through the

National Technical Information Service.

5285 Port Royal Road

Springfield, VA 22161

19. Security Classification (of this report)

Unclassified

20. Security Classification (of

this page)

Unclassified

21. No. of Pages

108

22. Price

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized

ii

DISCLAIMER

The contents of this report reflect the views of the authors, who are responsible for the facts

and the accuracy of the information presented herein. The contents do not necessarily reflect the

official views or policies neither of the Nebraska Department of Transportations nor the University

of Nebraska-Lincoln. This report does not constitute a standard, specification, or regulation. Trade

or manufacturers’ names, which may appear in this report, are cited only because they are

considered essential to the objectives of the report.

The United States (U.S.) government and the State of Nebraska do not endorse products or

manufacturers. This material is based upon work supported by the Federal Highway

Administration under SPR-P1(20) M111. Any opinions, findings and conclusions or

recommendations expressed in this publication are those of the author(s) and do not necessarily

reflect the views of the Federal Highway Administration.

iii

ACKNOWLEDGMENTS

This study was financially supported by the Nebraska Department of Transportation

(NDOT). Their support is gratefully acknowledged. The technical support and valuable

discussions provided by the Technical Advisory Committee (TAC) of this project is appreciated.

iv

TABLE OF CONTENTS

Page

TECHNICAL REPORT DOCUMENTATION PAGE ................................................................... i

DISCLAIMER ................................................................................................................................ ii

ACKNOWLEDGMENTS ............................................................................................................. iii

TABLE OF CONTENTS ............................................................................................................... iv

LIST OF TABLES ........................................................................................................................ vii

LIST OF FIGURES ..................................................................................................................... viii

1. INTRODUCTION .............................................................................................................. 1

1.1 Background ................................................................................................................ 1

1.2 Research Objective .................................................................................................... 3

1.3 Research Scope .......................................................................................................... 3

2. LITERATURE REVIEW ................................................................................................... 5

2.1 Introduction ................................................................................................................ 5

2.1 Research Literature .................................................................................................... 5

2.1.1 EPRI Report EL-6309 – Direct Embedment Foundation Research (1989) ... 5

2.1.2 Rojas-Gonzalez, DiGioia, Jr., and Longo (1991) .......................................... 7

2.1.3 Ong et al. (2006) .......................................................................................... 10

2.1.4 Bingel III and Niles (2009) .......................................................................... 11

2.1.5 Kandaris and Davidow (2015) ..................................................................... 12

2.2 Current State of Practice .......................................................................................... 13

2.2.1 Industry practice........................................................................................... 13

2.2.2 Direct embedment for cellular tower steel poles ......................................... 14

2.3 Proposed State of Practice ....................................................................................... 17

2.4 Examples of Design Specifications and Guidelines ................................................ 17

2.4.1 ASCE/SEI 48-11 – Design of Steel Transmission Pole Structures ............. 17

2.4.2 IEEE Guide for Transmission Structure Foundation Design and Testing ... 19

2.4.3 Omaha Public Power District (OPPD) technical specifications .................. 20

2.4.3 NDOT technical specifications .................................................................... 21

2.4.4 Other state DOT technical specifications .................................................... 22

2.5 Corrosion Considerations......................................................................................... 23

2.5.1 Parameters that influence underground metal corrosion ............................. 24

v

2.5.2 Qualitative evaluation of underground corrosion ........................................ 25

2.5.3 Quantitative evaluation of underground corrosion ...................................... 29

2.6 Summary .................................................................................................................. 35

3. HIGH MAST LOAD AND RESISTANCE ..................................................................... 36

3.1 Introduction .............................................................................................................. 36

3.2 Structural Loads ....................................................................................................... 36

3.2.1 General ......................................................................................................... 36

3.2.2 Dimensions and input parameters ................................................................ 37

3.2.3 Wind load ..................................................................................................... 37

3.2.4 Flexural response ......................................................................................... 40

3.2.5 Base reactions from structural analysis........................................................ 43

3.3 Foundation Resistance for Nebraska High Mast Towers ........................................ 43

3.3.1 General ......................................................................................................... 43

3.3.2 Loads on foundation .................................................................................... 44

3.3.3 Load carrying capacity ................................................................................. 44

3.3.4 Horizontal load carrying capacity ................................................................ 46

3.4 Summary .................................................................................................................. 48

4. FOUNDATION SYSTEMS ............................................................................................. 49

4.1 Introduction .............................................................................................................. 49

4.2 LPILE Analysis ........................................................................................................ 49

4.2.1 Input parameters........................................................................................... 49

4.2.2 Behavior of lateral piles with round concrete shaft and steel casing ........... 51

4.2.3 Effect of modulus of piles ............................................................................ 53

4.2.4 Parametric study for various foundation systems ........................................ 54

4.3 COMSOL Analysis .................................................................................................. 59

4.3.1 Input parameters........................................................................................... 59

4.3.2 Analysis (Clay Soil) ..................................................................................... 62

4.3.3 Analysis (Sandy soil) ................................................................................... 69

4.4 Selection Matrix based on Numerical Analysis ....................................................... 71

5. SITE CONSIDERATIONS AND CONSTRUCABILITY .............................................. 72

5.1 Introduction .............................................................................................................. 72

5.2 Construction Issues .................................................................................................. 72

5.2.1 Overview ...................................................................................................... 72

5.2.2 Earth formed ................................................................................................ 73

5.2.3 Polymer slurry .............................................................................................. 73

5.2.4 Culvert.......................................................................................................... 74

5.2.5 Permanent casing ......................................................................................... 74

5.2.6 Setting the pole ............................................................................................ 74

vi

5.2.7 Backfill ......................................................................................................... 75

5.3 Locality of Soil Conditions in Nebraska .................................................................. 75

5.5 Site Considerations and Steps for Corrosion Protection Strategies ......................... 78

5.6 Cost Comparisons .................................................................................................... 79

6. SUMMARY AND CONCLUSION ................................................................................. 81

6.1 Summary .................................................................................................................. 81

6.2 Conclusion ............................................................................................................... 82

6.3 Further Research ...................................................................................................... 84

REFERENCES ............................................................................................................................ 85

APPENDIX A ............................................................................................................................. 88

Qualifications and Submittals ........................................................................................ 88

Personnel Qualifications ....................................................................................... 88

Submittals ............................................................................................................. 88

Execution ....................................................................................................................... 89

Drilling Operations ............................................................................................... 89

Aggregate Placement ............................................................................................ 90

Concrete Placement .............................................................................................. 90

Direct Embedment Installation Record .......................................................................... 92

APPENDIX B .............................................................................................................................. 93

SPT Blow Counts ........................................................................................................... 93

Correction Factors of SPT Blow Count (N60) ................................................................ 93

Additional Correction Factors for Cohesionless Soils ................................................... 96

Combined Correction Factor for this Research ............................................................. 96

Internal Friction Angle ................................................................................................... 97

Cohesion ........................................................................................................................ 98

vii

LIST OF TABLES

Table Page

Table 2.1: Design Information of a 120-ft Direct Embed Steel Cellular Pole .............................. 15

Table 2.2: Design Information of a 140-ft Direct Embed Steel Cellular Pole .............................. 16

Table 2.3: Summary of State DOT Design Requirements for Light Pole Foundations ............... 23

Table 2.4: Underground Corrosion Likelihood Score Sheet (retrieved from DIPRA, 2018) ....... 27

Table 2.5: Underground Pipe Corrosion Consequence Score Sheet (retrieved from DIPRA, 2018

and Arriba-Rodriguez et al., 2018) ............................................................................................... 28

Table 2.6: DIPRA Design Decision Model (retrieved from DIPRA, 2018) ................................. 29

Table 2.7: Chemical and Physical Properties of the NBS Test Sites that represent Nebraska Soil

Groups (retrieved from Romanoff, 1957) ..................................................................................... 32

Table 2.8: Loss in weight and maximum penetration of wrought black ferrous metal (retrieved

from Romanoff, 1957) .................................................................................................................. 33

Table 2.9: Loss in weight and maximum penetration of 6-in. cast-iron pipe (retrieved from

Romanoff, 1957) ........................................................................................................................... 34

Table 2.10: Tentative NDOT Applications for Direct Embedment Foundations ......................... 35

Table 3.1: Calculated Base Reactions for a typical Nebraska 140-ft HMT.................................. 43

Table 3.2: Calculated Base Reactions for a typical Nebraska 120-ft and 140-ft HMT ................ 44

Table 4.1: Input Material Properties for LPILE Analysis............................................................. 51

Table 4.2: Parametric Study for Various Foundation Systems (140-ft Tower) ............................ 57

Table 4.3: Parametric Study for Various Foundation Systems (120-ft Tower) ............................ 58

Table 4.4: Geometric Properties for COMSOL model ................................................................. 60

Table 4.5: COMSOL Material Properties ..................................................................................... 62

Table 4.6: Selection Matrix for Various Direct Embedment Foundations ................................... 71

Table 5.1: Cost Comparisons between Conventional vs. Direct Embedment (USD) .................. 80

viii

LIST OF FIGURES

Figure Page

Figure 1.1: High Mast Lighting Tower in Milford, Nebraska (photo provided by NDOT) ........... 1

Figure 2.1: Rule-of-Thumb Design of Direct Embedment Foundation for Transmission Towers

(retrieved from the EPRI EL-6309 Report, 1989) .......................................................................... 6

Figure 2.2 Direct Embedment Foundation Model developed from the EPRI research (figure 3

directly retrieved from Rojas-Gonzalez et al. 1991) ....................................................................... 9

Figure 2.3: Applied Ground-Line Moment vs. Deflection at Ground-Line from the Full-Scale

Testing and Analysis (figure 7 directly retrieved from Rojas-Gonzalez et al. 1991) ..................... 9

Figure 2.4: Predicted vs. Applied Ground-Line Moment for Ground-Line Deflections of 0.5, 1.0,

and 2.0 inches (figure 8 directly retrieved from Rojas-Gonzalez et al. 1991) .............................. 10

Figure 2.5: Example Drawing of an Embedded Base Section for Galvanized Steel Poles (CAD

drawing provided by the Omaha Public Power District) .............................................................. 21

Figure 2.6: Rate of Corrosion of Metals buried under Soil (retrieved from de Arriba-Rodriguez et

al., 2018) ....................................................................................................................................... 25

Figure 2.7: Soil Groups of US and Dots indicating the NBS Test Sites (retrieved from Romanoff,

1957) ............................................................................................................................................. 31

Figure 3.1: Typical Moment Diagram from Analysis .................................................................. 42

Figure 4.1: Screen Capture of Figure 3.19 from LPILE Manual (Tab Sheet for Shaft Dimensions

of Drilled Shaft with Casing and Core to represent the Direct Embedment Foundation) ............ 50

Figure 4.2: Deflection of the embedded steel pole based on LPILE analysis .............................. 52

Figure 4.3: Shear Force and Bending Moment Profile for the embedded pole ............................ 53

Figure 4.4: Deflection of the embedded steel pole based on LPILE analysis .............................. 53

Figure 4.5: Shear Force and Bending Moment Profile for the embedded pole ............................ 54

Figure 4.6: Domain Geometry ...................................................................................................... 59

ix

Figure 4.7: Close Up of Tube, Fill, and Soil Domains ................................................................. 61

Figure 4.8: Bottom of Domain ...................................................................................................... 61

Figure 4.9: Elevation ..................................................................................................................... 61

Figure 4.10: Pole Von Mises Stresses (Clay) ............................................................................... 63

Figure 4.11: Translation in the direction of load .......................................................................... 64

Figure 4.12: Translation along the fill edge (Clay)....................................................................... 65

Figure 4.13: Normal Stresses in the direction of loading (Lfactor = 1, Clay) .............................. 65

Figure 4.14: Load-translation plot (Clay) ..................................................................................... 66

Figure 4.15: Boolean plot where plastic strains exist (red = yes, blue = no) ................................ 66

Figure 4.16: Effective Plastic Strain ............................................................................................ 67

Figure 4.17: Parametric Study Esoil (Lfactor = 1, Clay) .............................................................. 68

Figure 4.18: Parametric Variation Shaft Depth = 28-ft distance from bottom (Lfactor = 1, Clay)

....................................................................................................................................................... 68

Figure 4.19: Pole Bottom vs Effective Plastic Strain (Shaft Depth = 24 ft – Pole Bottom, Clay) 69

Figure 4.20: Load vs Max Translation (sandy soil) ...................................................................... 69

Figure 4.21: Translation in sandy soil........................................................................................... 70

Figure 4.22: Translation along fill for sandy soil ......................................................................... 70

Figure 5.1: Geologic Bed Rock Map of Nebraska (retrieved from Pabian, 1970) ....................... 76

1

1. INTRODUCTION

1.1 Background

High-Mast Tower (HMT) foundations have been traditionally designed and constructed

using cast-in-place foundation with anchor bolts that are used to secure the tower to the

foundation. This type of design requires a base plate that is welded to the tower shaft as shown

in Figure 1.1(a).

The Nebraska Department of Transportation (NDOT) has recently experienced issues

with stresses that this type of design presents at the anchor bolt/foundation or base plate/tower

shaft interface. This issue in worst cases may lead to a premature failure due to high-cycle

fatigue as shown in one of the towers at Milford, Nebraska that fell down during a winter snow

storm event in 2018.

(a) High Mast Tower Base Plate, Anchor,

Non-shrink Grout, and Cast-in-Place

Foundation

(b) High Mast Tower Failure (Milford,

Nebraska, January, 2018)

Figure 1.1: High Mast Lighting Tower in Milford, Nebraska (photo provided by NDOT)

There have been several research efforts in the past decade to evaluate the fatigue

behavior of these HMTs (Thompson 2011, Connor et al. 2012) to propose retrofit strategies that

2

could reduce wind-induced vibrations observed in these structures (Ahearn and Puckett, 2010).

Goode and van de Lindt (2007) developed a reliability-based design procedure for High Mast

Lighting Towers while Connor and Hodgson (2006) conducted field instrumentation and

conducted pluck tests on these structures to measure the dynamic characteristics.

While most of these previous studies have focused on the 100-120 ft tall structures, there

are limited or no research conducted for the substructure related to these towers. All research

whether related to load effects, mitigation of vibrations, and/or resistance of the pole-to-baseplate

connection always include the connection to a plate that is bolts for the foundation.

NCHRP Report 176 (2011) is one of the several studied that has examined the fatigue

resistance of typical pole-to-baseplate connections. The resistance is dependent upon the

connection geometry, relative tube-wall thickness to baseplate thickness, and specific of whether

the pole is attached via fillet welds or full penetration welds. Although weld details have been

significantly improved with research and associated specification updates, the joint remains a

potential weakness in the overall performance.

This research suggests that this connection may be completely eliminated thereby

obviating the need for the design of fatigue prone weldments. The joint is removed by directly

embedding the pole into the foundation. This approach is novel for the transportation industry

and could become an exceptional strategy for NDOT as well as other owners confronting failures

in their high-mast luminaire poles.

3

1.2 Research Objective

This research project aims to develop an alternative design for HMT foundations which

can eliminate fatigue-prone details associated with the pole-to-base plate connection which is the

primary location of failure. To address critical issues, the objectives are to:

1. Evaluate the various types of foundations used in other structures that are similar

in height and shape to the HMTs. This includes evaluating drilled shafts and

direct embedment foundations for Power Transmission Line Structures,

2. Evaluate the corrosive environment with steel pole structure being embedded

either in soil or concrete and propose mitigation measures for any corrosion

issues, and based on these findings,

3. Provide design and construction provisions that may integrate into NDOT

specifications for design and construction.

1.3 Research Scope

Three tasks support the objectives:

1. Review the literature review on foundations for structures that are similar in size and

shape with the high-mast towers used in Nebraska. One good example is the high-

tension power transmission line structures that use various types of foundations.

Literature review addressed single steel-shaft poles with direct embedment or drilled-

shaft foundations. Construction methods and corrosion protection strategies were

reviewed. NDOT was surveyed with respect to any problems associated with their

present drilled shafts. The shafts appear to be performing well. Note the shafts

involved with the new configuration experiences the same loads as any new

4

configuration. Although the latter might consider concrete or aggregate backfill with

in the foundation, presently only concrete is used.

2. Review site-specific corrosion investigations to evaluate the corrosivity of the soil

and provide mitigation strategies. This is possible through investigating the

resistivity, pH, chloride, sulfate, and oxygen content of the soils and water at potential

sites or in the concrete as indicated in the Caltrans (2018) report. Some NDOT

districts have corrosive soil, and because the foundations for these HMTs are close to

the surface where oxygen content may be higher, protective measures may be

required. The web soil survey system provided by the USDA Natural Resources

Conservation Service (NRCS, 2018) has soil maps and data available online for

Nebraska counties and can be used as a source for soil survey information. The most

typical protection for steel poles embedded in soil in a corrosive environment may be

achieved by providing by: 1) cathodic protection, 2) protective coatings (epoxy,

bituminous coating, etc.), 3) increasing cross-section area of steel, or 4) adding a

reinforced concrete jacket.

3. The final task includes use of the outcomes from the first two tasks and integrate

them into design and construction recommendations. All poles are assumed to be

galvanized. The recommendations informed the integration of materials into the

NDOT Inspection Guide for Installation of High-Mast Lighting and Sign Structures

(2008) manual and the NDOT Standard Specifications for Highway Construction

(2017) that can be applied statewide.

5

2. LITERATURE REVIEW

2.1 Introduction

This chapter provides a literature review focusing on steel poles with direct embedment

foundations or drilled shafts: 1) Academic and industry research results, 2) Drawings from the

industry practice, and 3) Design specifications, guidelines, or research reports published from

technical committees or societies were reviewed.

2.1 Research Literature

2.1.1 EPRI Report EL-6309 – Direct Embedment Foundation Research (1989)

This report delivers research results of numerical analysis and field load testing

conducted by the Electric Power Research Institute (EPRI) and the Empire State Electric Energy

Research Corporation (ESEERCO). The motivation of this study starts from introducing the “10

percent plus 2 feet” rule-of-thumb design (illustrated in Figure 2.1) for direct embedment

foundations of transmission towers. The report states that this rule-of-thumb works well for

wood poles but for steel and concrete poles with various backfill conditions, there is a need for a

better model representation and design criteria to resist the possible load conditions.

The study presents analytical models developed for direct embedment foundations and

field test results to predict the ultimate capacity and load-displacement behavior of direct

embedment foundations under full-scale load tests. Twelve full-scale direct embedment

foundations were loaded at seven different test sites. The pole height of the full-scale test

specimens varied between 65 to 115 ft with a maximum outer diameter varying from 28 to 38 in.

The embedment depth varied from 7.7 to 11.5 ft and the average hole diameter for the backfill

ranged from 41 to 57 in. Various backfills were used for the test specimens: 1) lightly

compacted silty clay, 2) loose crushed stone, 3) well-compacted silted clay, and 4) well-

6

compacted crushed stone. From each load tests, the subsurface investigation, foundation design,

pole installation and instrumentation, load testing, and data analysis were reported. The full-

scale foundation load test demonstrated that when loose or poorly compacted backfill was used,

the backfill dominated the load-deflection behavior with excessive ground-line deflection and

rotation at a small percentage of the ultimate capacity of the foundation. The test results also

indicated that the load-deflection response of foundations embedded into rock is significantly

different than soil-supported foundations. The direct embedment foundations with rock backfill

showed nearly linear moment deflection or rotation relationships.

Figure 2.1: Rule-of-Thumb Design of Direct Embedment Foundation for Transmission

Towers (retrieved from the EPRI EL-6309 Report, 1989)

The analytical models used in this study provided reasonable predictions compared to the

test results regarding the load-deflection response and the ultimate capacity of the direct

7

embedment foundations only when the foundations were soil-supported with well-compacted

backfill. This study also stated that analytical models used for drilled shaft foundations can also

be used for the direct embedment foundations when the backfill of direct embedment

foundations have similar or higher shear strength and stiffness characteristics than the native soil.

However, for the case where the backfill has lower shear strength and stiffness than the

surrounding soil, models used for drilled shafts do not work well for direct embedment

foundations because they do not consider variations in strength and stiffness in the radial

direction from the perimeter of the foundation. The study also indicated that the analytical

models used for drilled shafts typically only consider horizontal soil layering but a vertical layer

surrounding the pole must be modeled if a direct embedment foundation is used by assigning the

shear strength and stiffness parameters of either the backfill or the native soil.

For future research, this report asks the following questions to be answered: 1) What

backfill materials are most suitable for use in construction the direct embedment foundations? 2)

What amount of compaction is required for backfill to obtain satisfactory load-deflection

performance? 3)What types of compaction equipment are most suitable for compacting the

backfill in the relatively thin annulus surrounding a direct embedment foundation? and 4) Can

flowable backfill materials not requiring compaction such as cement-fly ash mixtures be

considered as material for backfill in the direct embedment foundations?

2.1.2 Rojas-Gonzalez, DiGioia, Jr., and Longo (1991)

This IEEE Transactions on Power Delivery article was prepared by two engineers from

GAI Consultants and one engineer from EPRI who were involved in the 1989 EPRI EL-6309

report and test program. The paper summarizes the results of the ten full-scale load tests and

analysis results using the direct embedment foundation model introduced in the EPRI EL-6309

8

report (see Figure 2.2). Test sites No. 3, 7, and 10 which had steel poles that are 115 ft high with

embedment lengths between 10.5 ft and 11.5 ft with an auger hole diameter of 52 and 54 in. with

well-compacted crushed stone or well-compacted sand with gravel as backfill material were

selected tests that were of interest due to the fact that Nebraska typically uses either a 120 ft or

140 ft high steel poles. However, HMT loads are typically lower than transmission structures.

Test site No. 3 with well-compacted crushed stone had some stiff clayey silt near the ground-line

and some sand below 3 ft to the bottom of the pole. Test site No. 7 had well-compacted crushed

stone mix for backfill material surrounded by very loose to loose gravelly silty sand from the

ground-line to 3 ft depth and medium dense sand and some gravel below 3 ft. Test site No. 10

had well-compacted crushed stone as backfill material surrounded by silty sand and dolomite

fragments in the upper 4 ft soil and Dolostone below 4 ft to the bottom of the pole. Two figures

from the original paper. Figure 2.3 shows how well the analytical model matches with the full-

scale test results. Figure 2.4 is the result of the analytical prediction versus the applied moment

measured at the test site with 0.5 in., 1 in., and 2 in. deflection at the ground-line. Figure 2.4 also

demonstrates that the prediction based on the analytical model matches well with the measured

applied moment at the full-scale test sites. Based on the model and the test results, at test site

No. 3, 0.5-in. ground-line deflection is observed at approximately 400 ft-kip moment while 1 in.

deflection is observed at approximately 600 ft-kip moment. At test site No. 7 and 10, ½ in.

ground-line deflection is observed at approximately 200 ft-kip moment, 1 in. deflection at 300 ft-

kip, and 2 in. deflection at 400 ft-kip moment. These test and analysis results can be served as a

reference for the High Mast Towers typically used in Nebraska (120 ft or 140 ft) which is

subjected to a similar load and ground-line displacement.

9

Figure 2.2 Direct Embedment Foundation Model developed from the EPRI research

(retrieved from Rojas-Gonzalez et al. 1991)

Figure 2.3: Applied Ground-Line Moment vs. Deflection at Ground-Line from the Full-

Scale Testing and Analysis (retrieved from Rojas-Gonzalez et al. 1991)

10

Figure 2.4: Predicted vs. Applied Ground-Line Moment for Ground-Line Deflections of

0.5, 1.0, and 2.0 inches (retrieved from Rojas-Gonzalez et al. 1991)

2.1.3 Ong et al. (2006)

This study was presented in the conference proceedings of the ASCE Electrical

Transmission and Substation Structures Conference and provided interesting results using

spectral analysis of surface waves to conduct the stability design of direct embedment pole

structures. The study highlights that the widely used Brom’s method for lateral stability would

only be valid for either cohesive or cohesionless soils. For that reason, when dealing with

natural soil which is mixed cohesive/cohesionless soil, empirical adjustments have to be made

and there is inconsistency with these theoretical geotechnical analyses. The study also stressed

out that the analytical models (MFAD by EPRI) require classical soil parameters as the friction

11

angle or cohesive strength and to obtain these properties, an intensive soil sampling should be

conducted at isolated points near the project site which is not easy and time consuming.

Therefore, this study offers an alternative method in obtaining the elastic material constants of

different soil layers by using mechanically induced stress waves and high-resolution transducers.

This research identifies that the geophysical technique introduced in this paper is low-cost and

the design approach using this geophysical method will provide reliable prediction of ground-

line deflection for directly embedded poles. This analytical method can determine pole stability

and soil strain by limiting the ground-line deflection based on easy to conduct, inexpensive, rapid

site characterization.

2.1.4 Bingel III and Niles (2009)

This paper included in the proceedings of the ASCE Electrical Transmission and

Substation Structures present methods for assessment and repair of steel tower and steel pole

foundations. The paper introduces both direct inspection procedures (visual assessment, physical

measurements, excavation, ultrasound measurements, electromagnetic acoustic transducer) and

indirect inspection methods (measurements of pH, Redox, soil resistivity, half-cell

measurements) to develop corrosion inspection on steel poles under soil. Common assessment

methods, possible corrosion ratings, and repair techniques are introduced in this summary paper

that can be adapted for the steel poles that will be directly embedded in soil. For example, the

study suggests using the “SSPC VIS2 Standard Method for Evaluating Degree of Rusting on

Painted Steel Surfaces” developed by the Society for Protective coatings that can be used to

classify the type of corrosion and severity for the samples collected and quantify the inspection

result following the standard. The study states that directly embedded poles often requires

necessary excavation for a shallow depth between 18 in. to 24 in. to allow the inspector to make

12

visual assessments and physical measurements to determine the amount of section loss caused by

corrosion. The study suggests when direct measurements or physical measurements from

excavation is not easy to conduct, that nondestructive testing methods using ultrasound or

electromagnetic acoustic transducers could also be an option to detect the corrosion on steel

members buried under ground. In addition, the study suggests that based on the direct or indirect

assessment, unless severe corrosion with metal loss, pitting, thinning, or perforation is observed,

if the corrosion rating is between low and moderate, the next inspection can be conducted in ten

years.

2.1.5 Kandaris and Davidow (2015)

This study was presented in the conference proceedings of the ASCE Electrical

Transmission and Substation Structures Conference and provided interesting results of the

survey conducted by an ad hoc task force team assembled by the Deep Foundation Institute with

experienced individuals from the electric power industry, utility consultants, utility companies

and academia. The conference proceedings paper summarizes the survey results of 45 questions

about the designer demographics, design approach, deep foundation design practices,

geotechnical exploration practices, reinforced concrete design practices, direct embedment pole

foundation design, alternate foundation types, foundation field testing and validation, and

construction considerations.

A total number of 22 surveys were collected which is a low number to generalize, but

there are some points we can still learn from this survey result that there is little consistency in

the foundation design of transmission lines for designers, consultants, and utilities. Many of the

utilities and consultants had their own internal design manuals due to the lack of a uniform

guidance or specifications. The top deflection of the foundation used as performance factors at

13

the ground-line varied between zero to six inches and rotation between zero to two degrees. Half

of the respondents used factored loads when evaluation foundation performance criteria, while

the rest used service loads or do not identify the load type. And, 73% of the designers stated that

they use some reduced foundation resistance due to the near surface soil conditions but no more

than one quarter agreed on the method for determining the reduced resistance. Regarding direct

embedment foundations, the survey results demonstrate that the backfill material was compacted

native soil (57%), engineered aggregate (79%), and concrete (74%). Most designers used

analytical (84%) and general practice (79%) methods for the embedment depth design. The

participants of the survey also addressed that 36% of the respondents have no limits to free fall

placement of concrete within drilled shafts and 54% do not allow cold joints within concrete

drilled shaft foundations.

2.2 Current State of Practice

2.2.1 Industry practice

The current state of US practice for high-mast tower foundations are drilled-shaft

foundations. These foundations consist of a drilled-shaft excavation with concrete, a rebar cage

containing longitudinal (vertical) and transverse steel, wiring conduit, and anchor bolts. The

anchor bolts are embedded with proper length and spacing to transfer structural loads to the

concrete. These bolts attach to a transverse plate that is welded to the pole.

The excavations are typically bored with a large excavator-mounted drill rig and

appropriately sized tooling for the shaft with diameters in 6-in increments typical. The anchor

bolts are set with a template. With the current state of practice, the foundation contractor never

handles the pole.

14

The pole erection is performed by another contractor who may self-perform or

subcontract. Frequently, the electrical contractor will perform or supervise this work and this

can be source malfunction as they typically do not perform foundation and structural work.

Owner inspections of in-service pole often reveal that bolts are loose, and likely, not tightened

properly during erection. Research is ongoing to address this important issue.

2.2.2 Direct embedment for cellular tower steel poles

Table 2.1 and Table 2.2 are the physical properties, effective projected area (EPA)

capacity of the pole, and maximum reactions at the base for a typical 120-ft direct embed steel

pole, and 140-ft direct embed steel pole, respectively, used for cellular towers in US designed by

Rohn design engineers for the TESSCO Technologies (https://www.tessco.com/). The

embedment depth ranges between 24 to 26 ft with aggregate backfill placed between the soil and

the steel pole for a 140-ft tower. The embedment depth for the 120-ft tower range between 19 to

23 ft with aggregate backfill. The 140-ft tower has a base shear ranging between 9.6 kips to 14.9

kips while the 120-ft tower has base shear ranging between 7.1 kips and 11.5 kips. These loads

are approximately two to three times the load observed in typical High Mast Towers with the

identical height of 120 and 140 ft. The auger diameter for the backfill ranged between 3.5 to 4.0

ft for 120-ft tower and 4.0 to 4.5 ft for 140-ft tower. The calculations shown in both tables have

notes that indicate that the tabulated EPA values are based on the assumption that 80% of the

total EPA is located at the top of the pole and the remaining is located 20-ft below the top. The

notes provided by the Rohn design engineers indicate that if all loads are located at the top of the

pole, the tabulated EPA capacity should be reduced by 20%.

https://www.tessco.com/

15

Table 2.1: Design Information of a 120-ft Direct Embed Steel Cellular Pole

(Information retrieved directly from https://resources.tessco.com/attachments/394772_ROHN-

120-ft-DEP-Design-Overview-193192.pdf)

https://resources.tessco.com/attachments/394772_ROHN-120-ft-DEP-Design-Overview-193192.pdf


16

Table 2.2: Design Information of a 140-ft Direct Embed Steel Cellular Pole

(Information retrieved directly from https://resources.tessco.com/attachments/304031_ROHN-

140-ft-DEP-Design-Overview-193193.pdf)



17

2.3 Proposed State of Practice

The proposed state of practice for high-mast towers is to adopt a similar system that the

electrical utility industry uses for transmission and distribution pole. Direct embedment

eliminates the fatigue prone weld details at the baseplate-to-tube connection. It also eliminates

the need to rebar cages, anchor bolt placement, bolt tightening, secondary issues such a gap

under the baseplate permitting animal nesting and moisture intrusion (but, Nebraska require a

non-shrink grout fill between baseplate and foundation).

This method requires the contractor to drill the same excavation but instead of setting a

rebar cage and an anchor bolt cage, the pole base positioned at the bottom of the shaft and the

pole is properly plumbed from the top. The lowest pole section will be approximately 15 to 20 ft

greater than the shaft depth. This section accommodates conduit holes and above-grade

handholes that are fabricated off site. The shaft is filled with either concrete or aggregate

backfill. The remaining sections of the pole attached to the bottom section with a slip-fit

connection that is standard practice. Various drilling and placement procedures are outlined in

the draft specification in Appendix A. The soil type and condition play a large role here.

The utility contractor installs the top sections and finishes the electrical work. Different

scenarios could be permitted, e.g., the pole might be assembled and then place in the shaft; this

would require proper crane and rigging.

2.4 Examples of Design Specifications and Guidelines

2.4.1 ASCE/SEI 48-11 – Design of Steel Transmission Pole Structures

The Structural Engineering Institute (SEI) within American Society of Civil Engineers

(ASCE) published a design guideline for steel transmission pole structures. In this document,

sections outline design and installation guidelines with commentary for directly embedded steel

18

transmission poles. The design guideline states in Section 9.4 of the guideline that the embedded

section shall be designed to resist the overturning moment, shear, and axial loads. In addition,

the guideline states that the length of the section of the pole below the ground line shall be

determined using a lateral resistance approach and the owner shall be responsible for supplying

the structural designer information regarding the embedment depth, allowable foundation

rotation, and design point of fixity of the embedded section. In the commentary of for Section

9.4, the document states that a directly embedded pole foundation typically is designed to

transfer overturning moments to the in-situ soil, rock, or backfill by means of lateral resistance

and axial loads can be resisted by a bearing plate installed on the base of the pole. If additional

bearing capacity is required, base-expanding devices can be installed at the bottom of the pole.

The commentary includes that the quality of backfill, method of placement, and degree of

compaction greatly affects the strength and rotation of the foundation system and, thereby, the

design of the embedded pole. In addition, the commentary states that the direct-embedded pole

foundations have become popular because of their relatively low installation cost. The guideline

also adds a commentary that buoyancy of the pole should be considered when using direct

embedded-poles where high water table is present. This document also provides in Section

11.5.2 some installation guidelines that the annular opening around the embedded pole shall be

backfilled with soil or concrete and that the soil shall be compacted in accordance with the

specification requirements. In the commentary of Section C11.5.2, the guideline provides

additional information addressing that care should be taken during the backfilling and

compaction process to prevent damage to the protective coating of the embedded pole section.

The commentary section for installation also addresses that the Line Designer should provide

specific recommendations and requirements for the type of backfill material and the method of

19

backfill placement to ensure that in-service behavior of the pole is in accordance with the design

assumptions made regarding pole rotation at the ground line.

2.4.2 IEEE Guide for Transmission Structure Foundation Design and Testing

This guide was jointly prepared by the Institute of Electrical and Electronics Engineers

(IEEE) Power Engineering Society Transmission and Distribution Committee and the American

Society of Civil Engineers (ASCE) Transmission Structure Foundation Design Standard

Committee. The guide states that direct embedment foundations which has been traditionally

used for wood pole foundations in distribution lines has recently increased in number for steel

and concrete pole foundations in transmission lines. This guide provides the definition of direct

embedment: “Foundations that are made by power augering a circular excavation in the ground

and inserting the pole (wood, steel, or concrete) directly into the excavation, and backfilling the

void between the pole and the sides of the excavation.” Then the guide highlights that the

quality of backfill, method of placement, and the degree of compaction strongly influences the

stiffness and strength of the direct embedment foundation. The guide also provides the principal

differences between direct embedment foundations and drilled shaft foundations. The guide

indicates that similar analytical models used in drilled shaft design can be used for direct

embedment foundations because the response of direct embedment foundations to compression,

uplift, and lateral loads is similar to that of drilled shafts. This is important for the present work

as well.

However, the major difference between the two types of foundations that this document

summarizes is that drilled shafts transfer loads directly into in-situ soil while direct embedment

foundations transfer loads to the backfill which in turn transfer the loads to the in-situ soil due to

the presence of backfill between the pole and the in-situ soil. The guide also introduces a

20

design/analysis model that can be used for direct embedment foundations which is the model

developed by the research conducted and summarized on the EPRI EL-6309 Report introduced

in the research literature section.

The following are some of the important aspects highlighted in the report: 1) Backfill

clearly constitutes an important element of the direct embedment foundation, 2) Granular

backfill can be readily compacted and is generally preferable to a cohesive soil, 3) To obtain

proper compaction, the backfill should be placed in layers of 6 in. or less and compacted to the

specified density, 4) The uplift capacity of direct embedment foundations is related to the quality

of the backfill and the adhesive and friction forces that can be mobilized at the structure-backfill

interface or at the backfill-in-situ soil interface, 5) For compression loads, significant end-

bearing capacity can only be achieved if the direct embedded pole is closed with a base plate.

2.4.3 Omaha Public Power District (OPPD) technical specifications

The Omaha Public Power District (OPPD) published a 17-page document with technical

specifications for galvanized steel transmission structures that includes design and construction

requirements. The specification addresses that all stress calculations shall be based on an elastic

analysis in accordance with ASCE/SEI 48-11. The following is to highlight some of the design

requirements (Section 3) for embedded poles listed in this specification: 1) Embedded poles shall

continue the taper at ground line to a point 6 feet below ground line. From this point to the

bottom of the section, the taper may continue or the diameter may be held constant. No reverse

tapers will be allowed. 2) The base section of all embedded poles shall have a maximum

projection 30 ft above the ground line unless otherwise noted on the drawings. 3) The base of all

embedded poles shall have a minimum projection of 10 ft above the ground line unless otherwise

noted on the drawings. For construction requirements, the specification states that direct

21

embedded poles shall have a minimum of 3/16 in. thick ground sleeve extending above and

below ground line as shown on the drawings. In addition, the document states that all direct

embedded galvanized steel poles shall be coated with a minimum 16 mil dry thickness of two

component hydrocarbon extended polyurethane coating that is resistant to ultraviolet light and

conform to ASTM G14-77. Figure 2.5 shows an example of the embedded base section from a

typical galvanized steel pole drawing provided by OPPD.

Figure 2.5: Example Drawing of an Embedded Base Section for Galvanized Steel Poles

(CAD drawing provided by the Omaha Public Power District)

2.4.3 NDOT technical specifications

The NDOT Standard Specifications for Highway Construction (2017) Section 407

contains material requirements, construction methods, method of measurement, and basis of

payment for pole and tower foundations. Either a concrete foundation or a power installed

foundation are the options listed in the technical specifications. The size of the concrete

foundation should be sufficient to include ground rods, reinforcing steel, anchor bolts, conduit

22

entrance bends, and a spare conduit bend. It is recommended that the contractor shall obtain soil

data and design and construct the tower foundation if the foundation details are not shown in the

contract. NDOT specifications states that the concrete foundations for the tower installations

shall be constructed according to the following steps: a) All foundation excavations shall be free

or loose dirt, b) All concrete shall be Class 47B-3000 (47B-20), c) The anchor bolt pattern shall

be centered in the foundation, and 3) The contractor shall perform all excavations, backfilling,

and placing of reinforcing steel and concrete in accordance with Sections 702 (Excavation for

Structures), 704 (Concrete Construction), and 707 (Reinforcement) of NDOT Standard

Specifications for Highway Construction.

2.4.4 Other state DOT technical specifications

Table 2.3 was developed by the research team currently working with the Alaska

Department of Transportation (Alaska DOT) on evaluation of light pole foundation embedment.

The table summarizes the current State Department of Transportation (State DOT) practices for

light pole foundation designs. Delaware and Illinois are the only states that require a special

requirement for these foundation designs to conduct further structural analysis or soil analysis

coordinated with geotechnical engineers. Connecticut, Maine, and Oregon use the AASHTO

LRFD Specifications for Structural Supports for Highway Signs, Luminaires, and Traffic Signals

(AASHTO LRFD LTS) (2015) for their foundation design.

23

Table 2.3: Summary of State DOT Design Requirements for Light Pole Foundations

State DoT Design Guidelines on Luminaire

Foundation Remarks

Connecticut AASHTO LRFD LTS (2015)

Standard drawings are provided.

https://portal.ct.gov/DOT/CONNDOT/SECTION-

M15#M.15.04

Delaware

Considers structural analysis for cases of

nonstandard longer arm or pole height.

Foundation design shall be coordinated

with Delaware DOT’s Geotechnical

engineer.


https://deldot.gov/Business/drc/pdfs/traffic/Lightin

gPolicy.pdf?cache=1599176213610

Illinois

High-mast lighting requires special

designs and soil analyses. A 4ft diameter

reinforce concrete foundation is

recommended

https://idot.illinois.gov/Assets/uploads/files/Doing

-Business/Manuals-Split/Design-And-

Environment/BDE-Manual/Chapter 56 Highway

Lighting.pdf

Kansas

Special condition for non-typical soil

condition (frozen, saturated or soft soils)

– engineer’s directions to remedy

unsuitable foundations shall be followed.

https://www.wycokck.org/WycoKCK/media/Publi

c-Works/Engineering/Documents/Technical-

Provisions-2008-Edition.pdf

Maine

PE shall design foundations

AASHTO LRFD LTS (2015)


https://www.maine.gov/mdot/contractors/publicati

ons/standardspec/docs/2014/StandardSpecification

-full.pdf

Michigan Standard drawings are provided

https://mdotjboss.state.mi.us/TSSD/getCategoryD

ocuments.htm?categoryPrjNumbers=1403886,202

8779,1403887,1403888,1403889,1403890,179778

6&category=Traffic%20Signing

Minnesota Standard drawings are provided. https://www.dot.state.mn.us/trafficeng/lighting/20

10_Roadway%20Lighting_Design_Manual2.pdf

New

Hampshire

Precast or cast in place bases.


https://www.nh.gov/dot/org/projectdevelopment/hi

ghwaydesign/specifications/documents/2016NHD

OTSpecBookWeb.pdf

Ohio Standard drawings are provided.

http://www.dot.state.oh.us/Divisions/Engineering/

Roadway/DesignStandards/traffic/SCD/Document

s/HL_02011_2020-07-17.pdf

Oregon

TM653 for typical condition and TM650

for foundations to resist larger design

loads.

AASHTO LRFD LTS (2015)

https://www.oregon.gov/ODOT/Engineering/Basel

ineReport/TM653.pdf

Texas Standard drawings are provided. ftp://ftp.dot.state.tx.us/pub/txdot-

info/cmd/cserve/standard/traffic/ridfn11.pdf

2.5 Corrosion Considerations

This discussion is relative to metals in direct contact with the soil. In the case of directly

embedded HMTs, the surrounding backfill is concrete or aggregate. For concrete, clearly a large

protective layer exists and this is similar as for assets such as drilled-shaft foundation, concrete

slabs on grade, abutments, and so forth. The considerations in this case are similar to protecting

https://portal.ct.gov/DOT/CONNDOT/SECTION-M15#M.15.04

https://portal.ct.gov/DOT/CONNDOT/SECTION-M15#M.15.04

https://deldot.gov/Business/drc/pdfs/traffic/LightingPolicy.pdf?cache=1599176213610

https://deldot.gov/Business/drc/pdfs/traffic/LightingPolicy.pdf?cache=1599176213610

https://idot.illinois.gov/Assets/uploads/files/Doing-Business/Manuals-Split/Design-And-Environment/BDE-Manual/Chapter%2056%20Highway%20Lighting.pdf




https://www.wycokck.org/WycoKCK/media/Public-Works/Engineering/Documents/Technical-Provisions-2008-Edition.pdf



https://www.maine.gov/mdot/contractors/publications/standardspec/docs/2014/StandardSpecification-full.pdf



https://mdotjboss.state.mi.us/TSSD/getCategoryDocuments.htm?categoryPrjNumbers=1403886,2028779,1403887,1403888,1403889,1403890,1797786&category=Traffic%20Signing




https://www.dot.state.mn.us/trafficeng/lighting/2010_Roadway%20Lighting_Design_Manual2.pdf

https://www.dot.state.mn.us/trafficeng/lighting/2010_Roadway%20Lighting_Design_Manual2.pdf

https://www.nh.gov/dot/org/projectdevelopment/highwaydesign/specifications/documents/2016NHDOTSpecBookWeb.pdf



http://www.dot.state.oh.us/Divisions/Engineering/Roadway/DesignStandards/traffic/SCD/Documents/HL_02011_2020-07-17.pdf



https://www.oregon.gov/ODOT/Engineering/BaselineReport/TM653.pdf

https://www.oregon.gov/ODOT/Engineering/BaselineReport/TM653.pdf

ftp://ftp.dot.state.tx.us/pub/txdot-info/cmd/cserve/standard/traffic/ridfn11.pdf

ftp://ftp.dot.state.tx.us/pub/txdot-info/cmd/cserve/standard/traffic/ridfn11.pdf

24

reinforcement steel with a coating. For aggregate backfill, the site soil is not in direct contact;

however, the groundwater and the associated chemistry is likely best characterized by this soil.

It is likely that concrete will be the typical solution, and that the aggregate fill is the

exception. Moreover, concrete backfill could be explicitly specified in corrosive locations. The

discussion below helps to provide a means for identifying such locations. (this should be

consistent with NDOT methods used for other assets as well)

2.5.1 Parameters that influence underground metal corrosion

Metals buried under soil behave completely different from metals in the atmosphere.

Because the underground environment changes constantly depending on the soil condition, the

electrochemical process (oxidation and oxygen reduction; anodic and cathodic reactions) can

differ for the underground environment even when the locations considered are geographically

close. The soil texture (silt, sand, clay contents), moisture content, oxygen concentration, redox

potential (degree of aeration in soil), pH level, resistivity, ion contents (specifically, the amount

of aggressive ion salts such as chloride and sulfate contents), and bacteria level of the soil are all

important parameters that control the rate of corrosion of metal in soil.

It is well known no single parameter listed above that affects the rate of corrosion of

buried metal but rather multiple of them interacting together through an iterative process that

affects the metallic structures buried underground. For this reason, measurements of a single

parameter (for example, only measuring the soil resistivity) should not be used to plan corrosion

mitigation plans and design for corrosion resistance of a buried metal structure. As shown in

Figure 2.6 (de Arriba-Rodriguez et al., 2018), any of the parameters listed above can affect the

rate of corrosion.

25

Figure 2.6: Rate of Corrosion of Metals buried under Soil (retrieved from de Arriba-

Rodriguez et al., 2018)

2.5.2 Qualitative evaluation of underground corrosion

There are multiple qualitative and quantitative methods that can be used to make the

decision to select which corrosion protection method will be used or determine the design values

(coating thickness, metal thickness for corrosion protection, etc.) for steel structures being

embedded in soil. Again, with the understanding no one single parameter but multiple

parameters that affect the underground corrosion, NDOT can adapt or modify one of these

existing qualitative or quantitative methods that consider various factors in evaluating the

underground corrosion steel embedded poles.

For example, for a qualitative evaluation, the American Water Works Association

(AWWA) uses a point system that takes account for soil resistivity, pH, Redox Potential (mV),

Sulphide, and Moisture levels. The Ductile Iron Pipe Research Association (DIPRA) developed

a design decision model based on the likelihood, considering various parameters that AWWA

26

listed, and consequence (pipe diameter, construction repair cycles, depth of the embedment, and

alternate water supply conditions) they selected for underground pipes. Table 2.4, Table 2.5,

Table 2.6 and are a score sheet, consequence score sheet, and the corrosion protection

recommendations, respectively, that DIPRA recommends for use in qualitative design of

underground pipes. Based on this qualitative design decision model, the engineer can select

either which corrosion protection method (standard protection, polyethylene encasement,

metallized zinc-coating, or cathodic protection) to use in underground applications.

NDOT can possibly use a similar method to qualitatively determine the level of corrosion

protection for the HMT foundation directly embedded using aggregate backfill. The parameters

used by AWWA and DIPRA for the likelihood (Table 2.4) of metals being corroded under soil is

similar for the HMT directly embedded. For example, the California Department of

Transportation (CALTRANS) classifies every site as corrosive or non-corrosive site for their

projects. They define a site to be corrosive if one or more of the following conditions exist for

the representative soil at the site: Chloride concentration is 500 ppm or greater, sulfate

concentration is 2,000 ppm or greater, or the pH is 5.5 or less. If the site is defined to be

corrosive, corrosion mitigation is required.

27

Table 2.4: Underground Corrosion Likelihood Score Sheet (retrieved from DIPRA, 2018)

Likelihood Factor Points

Maximum

Possible

Points

Soil Resistivity < 500 ohm-cm 30 30

≥ 500-1000 ohm-cm 25

> 1000-1500 ohm-cm 22

> 1500-2000 ohm-cm 19

> 2000-3000 ohm-cm 10

> 3000-5000 ohm-cm 5

> 5000 ohm-cm 0

Chlorides > 100 ppm = positive 8 8

50 – 100 ppm = trace 3

< 50 ppm = negative

0

Moisture Content > 15% = wet 5 5

5 – 15% = moist 2.5

< 5% = dry 0

Ground

Water Influence

Pipe below the

water table at any time 5 5

pH pH 0 – 4 4 4

pH > 4 – 6 1

pH 6 – 8, with sulfides

and low or negative redox

4

pH > 6 0

Sulfide Ions positive (≥ 1 ppm) 4 4

trace (> 0 and < 1 ppm) 1.5

negative (0 ppm) 0

Redox = negative 2 2

Potential = positive 0 – 100 mv 1

= positive > 100 mv 0

Bi-metallic

Considerations

Connected to noble metals

(e.g. copper) – yes

2 2

Connected to noble metals

(e.g. copper) – no

0

TOTAL POSSIBLE POINTS 60

Known Corrosive

Environments

Cinders, Mine Waste, Peat Bog, Landfill, Fly Ash, Coal 21

Soils with known corrosive environment shall be assigned 21 points or the total of points for likelihood factors,

whichever is greater

Similar to Table 2.4 of DIPRA, CALTRANS would measure the soil resistivity and if the

soil has a minimum resistivity less than 1,000 ohm-cm, they classify that the soil has high

possibilities to transport soluble salts and is susceptible for corrosion activities to take place.

When the minimum resistivity is less than 1,000 ohm-cm, chemical testing is further required to

evaluate the chloride and sulfate level in the soil sample. The threshold values that CALTRANS

28

uses are comparable with the point system used by DIPRA (Table 2.4) and indicates the cases

with high points (more likely corrosion will take place). (CALTRANS, 2018)

However, the parameters listed in the consequence (Table 2.5) for pipe design should be

modified for HMTs. Possible consequence factors would be the embedment depth, pole

diameter below the ground-line, and access availability for potential repair work. If the

embedment depth is deep, or the access availability for potential repair work will be low, or the

pole diameter below the ground-line is large, the consequence scores will be high. The possible

corrosion mitigation methods in Table 2.5 should also be modified for other methods that can be

applied for HMTs.

Table 2.5: Underground Pipe Corrosion Consequence Score Sheet (retrieved from DIPRA,

2018 and Arriba-Rodriguez et al., 2018)

Consequence Factor Points Maximum

Possible Points

Pipe in Service 3” to 24 “ 0 22

30” to 36” 8

42” to 48” 12

54” to 64” 22

Location:

Routine (Fair to good access, minimal traffic

and other utility consideration, etc.)

0 20

Construction-

Repair

Considerations

Moderate (Typical business and residential

areas, some right of way limitations, etc.)

8

Difficult (Subaqueous crossings, downtown

metropolitan business areas, multiple utilities

congestion, swamps, etc.)

20

Depth of Cover 0 to 10 feet depth 0 5

Considerations > 10 to 20 feet depth 3

> 20 feet depth 5

Alternate Alternate supply available - no 0 3

Water Supply Alternate supply available – yes 3

TOTAL POSSIBLE POINTS 50

29

Table 2.6: DIPRA Design Decision Model (retrieved from DIPRA, 2018)

Likelihood (𝒙) Consequences (C) Proposed Action

< 10 Any Standard protection

10-20 < 30 Standard protection

1035.7𝑥−1.05> C > 240.3𝑥−0.7 Polyethylene Encasement (PE)

> 1035.7𝑥−1.05 PE with Joint Bonds

20-35 < 25 PE

1596.1𝑥−1.08> C > 1035.7𝑥−1.05 PE with Joint Bonds

> 1596.1𝑥−1.08 PE with Metallized Zinc Coating

35-40 < 30 PE with Joint Bonds

1177.8𝑥−0.89> C > 1596.1𝑥−1.08 PE with Metallized Zinc Coating

> 1177.8𝑥−0.89 PE with Cathodic Protection

40-45 < 1177.8𝑥−0.89 PE with Metallized Zinc Coating

> 1177.8𝑥−0.89 PE with Cathodic Protection

45-50 Any PE with Cathodic Protection

The most typical protection for a steel pole embedded with aggregate backfill in a

corrosive environment may be achieved by providing either one of the followings or a

combination of: 1) protective coatings (epoxy, bituminous coating, etc.), 2) cathodic protection,

3) increasing cross-section area of steel, or 4) reinforced concrete jacket. Again, the concrete

backfill is the best solution in corrosive areas and its use parallel that of other NDOT assets in

such areas.

2.5.3 Quantitative evaluation of underground corrosion

Quantitative methods exist for corrosion assessment for underground steel structures

whether to decide the structural dimensions that guarantee the service life of the structure being

buried in soil, field data that provides the information of soil condition (soil type, aeration level,

moisture level, pH level, soil resistivity, ion contents, and any bacteria contents), the amount of

steel loss in weight (due to corrosion) and the maximum penetration depth (pit depth in terms of

mils/years) can be used. Fortunately, the most complete quantitative study around the world was

conducted in US by the National Bureau of Standards (NBS; now National Institute of Standards

30

and Technology) and this is the most complete database that is still used these days although the

study may have limitations. NBS (now NIST) conducted a 45-year (1910-1955) study around

the entire US with thousands of steel samples buried and exposed in carefully selected

underground sites. The test sites and the 45-year corrosion study results are summarized by

Romanoff (1957). A total number of 128 sites were selected in US which represents 95 types of

soils. As shown in Figure 2.7 (Romanoff, 1957), Nebraska has four different types of soil

groups: 1) Prairie Soil, 2) Chernozen Soil, 3) Nebraska Sand Hills (no other locations in US have

this soil type), and 4) Dark Brown Soil.

Within the 128 sites, 10 sites can be selected to represent the Nebraska soil types. The

Knox silt loam and the Wabash silt loam from Omaha, Nebraska, Lindley silt loam from Des

Moines, Iowa, Marshall silt loam from Kansas City, Missouri, and the Muscatine silt loam from

Davenport, Iowa may represent the Prairie soil type which can be found in most of the eastern

part of Nebraska. The Fargo clay loam from Fargo, North Dakota may represent the Chernozen

soil type in the center Nebraska. Unfortunately, there was no test site in the NBS study that can

represent the Nebraska Sand Hills region. The unidentified silt loam from Denver, Colorado,

Billings silt loam from Grand Junction, Colorado can represent the Dark Brown Soils found in

the western Nebraska. The drainage, soil resistivity, pH, chemical composition, temperature,

annual precipitation, moisture level, air-pore space, apparent specific gravity, and volume

shrinkage of the test sites are reported for these sites. The sand, silt, clay components were also

measured for some of these sites. The loss in weight and the maximum penetration depth were

reported for an average of two specimens per site.

31

Figure 2.7: Soil Groups of US and Dots indicating the NBS Test Sites (retrieved from

Romanoff, 1957)

Table 2.7 shows the chemical and physical properties of the soils at the NBS test sites.

Table 2.8 shows the loss in weight and the maximum penetration depth measured during the test

period. If NDOT would like to make engineering decisions for the dimensions of the High Mast

Towers that will be buried under soil or coating thickness required for a desired service level

Table 2.7, Table 2.8, and Table 2.9 could be a reference in providing quantitative calculations

based on the extensive field observation data produced by NIST.

32

Table 2.7: Chemical and Physical Properties of the NBS Test Sites that represent Nebraska Soil Groups (retrieved from Romanoff, 1957)

Soil

Location

Inter-

nal

drain-

age

of test

site

Resist

-ivity

at

60℉

(ohm-

cm)

pH

Composition of water extract, mg-eq per 100 g of soil Mean

Tem-

pera-

ture

(℉)

An-

nual

preci-

pita-

tion

(in.)

Mois-

ture

equiv-

alent

(%)

Air-

pore

space

(%)

Appar

-ent

speci-

fic

gravit

y

Vol-

ume

shrin

k-age

(%)

Test

Site

No.

Type Total

Acidit

y

Na +

K as

Na

Ca Mg CO3 HCO3 CI SO4

8 Fargo

clay loam

Fargo,

North Dakota P 350 7.6 A 1.42 1.72 2.55 0.00 0.71 0.01 4.43 39 24 37.0 8.7 1.56 21.0

18 Knox

silt loam

Omaha,

Nebraska G 1,410 7.3 1.4 0.27 0.63 0.20 0.00 0.94 0.00 0.25 50.6 27.8 28.4 16.6 4.26 1.3

19 Lindley

silt loam

Des Moines,

Iowa G 1,970 4.6 10.9 0.38 0.32 0.41 0.00 0.46 0.03 0.46 49.5 32.0 28.4 3.9 4.76 11.8

21 Marshall

silt loam

Kansas City,

Missouri F 2,370 6.2 9.5 - - - - - - - 54.4 37.1 31.2 10.8 1.66 6.5

30 Muscatine

silt loam

Davenport,

Iowa P 1,300 7.0 2.6 0.32 0.65 0.40 0.00 0.71 0.09 0.24 49.9 32.4 29.4 7.2 1.81 7.5

44 Wabash

silt loam

Omaha,

Nebraska G 1,000 5.8 8.8 1.05 1.08 0.66 0.00 1.97 0.82 0.41 50.6 27.8 31.2 7.2 1.55 6.0

46 Unidentified

silt loam

Denver,

Colorado G 1,500 7.0 - - - - - - - - 50.0 14.4 7.6 23.2 - 0

101 Billings

silt loam

Grand

Junction, CO F 261 4.5 A 5.21 19.24 1.43 0.00 0.66 1.56 22.48 52.0 8.8 30.0 - - -

102 Billings

silt loam

Grand

Junction, CO F 103 7.3 A 22.63 16.56 3.85 0.00 0.56 4.67 36.82 52.0 8.8 20.4 - - -

103 Billings

silt loam

Grand

Junction, CO F 81 7.3 A 22.01 13.32 2.00 0.00 0.18 11.09 25.70 52.0 8.8 30.6 - - -

33

Table 2.8: Loss in weight and maximum penetration of wrought black ferrous metal (retrieved from Romanoff, 1957)

Soil

Location

Durat-

ion

of

expos-

ure

(years)

Loss in weight (oz/ft2) Maximum penetration (mils)

1.5-in. pipe 3-in. pipe 1.5-in. pipe 3-in. pipe

Test

Site

No.

Type of Soil

Open-

hearth

iron

Wrou-

ght

iron

Besse-

mer

steel

Besse-

mer

steel

(scale

free)

Wrou-

ght

iron

Open-

hearth

steel

Besse-

mer

steel

Open-

hearth

Steel

with

0.22

perce-

nt Cu

Open-

hearth

iron

Wrou-

ght

iron

Besse-

mer

steel

Besse-

mer

steel

(scale

free)

Wrou-

ght

iron

Open-

hearth

steel

Besse-

mer

steel

Open-

hearth

Steel

with

0.22

perce-

nt Cu

8 Fargo

clay loam

Fargo,

North Dakota

1.1

3.8

5.8

7.7

9.9

11.8

0.7

1.9

3.2

3.3

5.1

8.4

0.7

1.9

3.2

3.3

5.1

6.9

0.7

2.0

2.9

3.2

4.6

7.7

0.8

2.0

2.6

3.2

4.6

6.5

0.7

1.7

2.9

4.1

5.2

8.7

0.6

1.7

3.1

3.5

5.3

7.9

0.8

1.9

3.3

4.2

5.6

8.3

0.7

1.8

3.3

4.3

5.6

8.8

44

46

62

62

74

100

30

36

52

52

66

76

38

44

52

66

61

74

30

37

54

52

67

58

38

38

68

62

63

83

43

51

77

86

93

92

47

48

56

75

72

110

62

57

70

94

75

127

18 Knox

silt loam Omaha, Nebraska

1.2

3.8

5.8

7.7

9.8

11.7

0.4

1.2

2.3

1.8

2.8

3.0

0.7

1.8

4.0

3.3

3.5

2.7

0.4

1.9

3.2

2.9

3.2

3.1

0.6

1.6

2.6

2.0

3.3

2.6

0.4

1.3

2.9

2.2

3.0

2.0

0.3

1.2

2.7

1.9

3.4

2.7

0.5

1.2

3.2

2.3

3.1

2.4

0.5

1.7

2.6

2.1

2.9

3.9

27

40

71

55

46

52

20

40

72

37

46

42

19

48

71

40

51

41

14

34

67

42

57

38

24

34

64

56

54

41

28

52

66

57

64

70

16

43

62

42

50

44

28

37

80

50

46

44

19 Lindley

silt loam Des Moines, Iowa

1.1

3.7

5.7

7.6

9.7

11.6

0.6

2.0

2.2

2.5

3.0

2.9

0.8

1.5

3.0

3.0

3.2

3.5

0.7

2.2

2.2

2.7

3.2

3.8

0.7

2.2

2.3

2.8

2.9

3.4

0.6

2.3

2.3

2.4

3.1

3.2

0.7

2.3

2.3

2.4

2.9

3.4

0.8

1.8

2.0

3.0

3.1

3.3

0.8

2.0

2.1

2.5

3.0

3.5

38

38

46

44

62

56

24

45

50

40

51

62

26

50

48

46

61

71

24

38

48

40

55

66

28

36

58

56

56

66

25

46

59

58

78

85

32

59

58

56

65

60

38

50

50

66

68

63

21 Marshall

silt loam

Kansas City,

Missouri

1.5

4.0

6.0

2.0

2.5

4.2

2.1

2.6

4.4

2.1

2.9

5.0

2.1

2.4

4.7

2.2

2.8

4.8

2.0

2.9

5.1

2.0

2.9

4.6

2.1

2.5

4.6

< 10

61

71

< 10

40

52

< 10

50

60

< 10

63

58

< 10

60

60

< 10

39

59

< 10

48

66

< 10

41

60

30 Muscatine silt

loam Davenport, Iowa

1.1

3.6

5.7

8.2

11.6

17.0

0.9

1.1

1.8

4.1

5.2

6.1

0.9

1.2

2.3

4.7

5.6

5.9

1.1

1.2

2.6

4.1

4.8

5.7

0.9

1.2

2.0

4.2

5.3

5.4

0.9

1.4

2.5

4.7

6.3

6.0

1.1

1.3

2.7

4.0

5.8

6.0

1.1

1.4

2.5

4.4

5.6

6.9

1.0

1.2

2.4

3.8

5.2

6.4

24

< 20

< 20

46

54

50

< 10

< 20

< 20

36

51

44

12

< 20

24

33

58

52

10

< 20

16

34

51

42

12

< 20

28

62

53

53

16

< 20

28

35

54

64

16

< 20

27

48

63

76

18

< 20

28

31

66

65

44 Wabash

silt loam

Omaha,

Nebraska

1.1

3.6

5.7

7.6

11.6

0.3

1.4

2.3

1.7

2.9

0.6

1.8

2.2

2.3

4.1

0.4

2.0

2.3

2.2

4.7

0.5

1.8

2.4

2.0

3.5

0.4

1.4

2.1

1.9

3.4

0.3

1.4

2.2

2.1

2.8

0.4

1.2

2.0

2.0

3.4

0.4

1.3

2.0

2.1

3.2

38

78

70

72

87

36

43

52

49

56

28

54

51

62

63

32

55

66

50

69

26

46

56

56

65

39

44

72

50

58

32

44

60

62

82

38

50

68

88

74

46 Unidentified

silt loam

Denver,

Colorado

1.5

4.0

5.1

8.0

10.2

12.0

0.8

2.7

2.9

5.6

4.0

4.0

1.3

3.2

2.8

6.2

4.7

5.1

1.2

2.9

3.1

5.2

4.8

4.5

0.9

2.6

3.0

5.7

4.1

4.4

1.0

2.4

2.8

5.7

4.4

4.7

1.1

2.6

3.0

5.9

3.6

4.3

1.2

2.7

2.6

5.8

4.3

4.8

1.2

3.2

3.2

6.7

3.9

4.8

57

80

68

60

74

48

54

64

65

80

95

62

55

79

66

108+

68

64

54

82

54

118+

83

104

50

66

68

69

82

77

40

58

46

68

66

62

56

106

96

136

84

114

79

110

96

134

80

80

101

Billings

silt loam

(low alkali)

Grand Junction,

CO

1.9

4.1

9.3

- - - -

5.2

8.8

9.4

3.9

7.5

10.5

3.9

7.2

9.1

- - - - -

66

94

95

70

116

131

60

94

86

-

102

Billings

silt loam

(moderate

alkali)

Grand Junction,

CO

1.9

4.1

9.3

- - - -

5.1

10.2

16.1

3.9

9.4

18.3

3.9

7.2

9.1

- - - - -

37

80

93

42

102

124

26

72

95

-

103

Billings

silt loam

(high alkali)

Grand Junction,

CO

1.9

4.1

9.3

- - - -

5.0

10.4

21.3

3.7

11.2

18.8

3.6

10.1

17.8

- - - - -

48

86

136

62

88

190

37

66

192

-

34

Table 2.9: Loss in weight and maximum penetration of 6-in. cast-iron pipe (retrieved from Romanoff, 1957)

Soil

Location

Durat-

ion

of

expos-

ure

(years)

Loss in weight (oz/ft2) Maximum penetration (mils)

Centrifugal process Vertical cast in sand mold Centrifugal process Vertical cast in sand mold

Test

Site

No.

Type of Soil

deLa-

vaud

C

deLa-

vaud

CCd

Mono-

Cast

I

North-

ern

Ore

L

South-

ern

Ore

Z

South-

ern

Ore

A

deLa-

vaud

C

deLa-

vaud

CCd

Mono-

Cast

I

North-

ern

Ore

L

South-

ern

Ore

Z

South-

ern

Ore

A

8 Fargo

clay loam

Fargo,

North Dakota

1.1

3.8

5.8

7.7

9.9

11.8

0.8

1.9

3.4

5.6

8.4

16.8

- -

0.7

2.7

5.0

5.0

10.5

20.3

2.1

5.4

5.7

7.5

9.6

30.8

-

< 10

34

64

107

142

179

- -

59

76

91

85

217

240

73

61

81

78

169

239

-

18 Knox

silt loam Omaha, Nebraska

1.2

3.8

5.8

7.7

9.8

11.7

0.1

0.6

2.3

2.0

3.5

5.5

- -

-

1.1

3.3

3.2

4.8

2.7

-

0.9

5.1

2.7

4.6

4.8

-

< 10

38

76

< 20

69

85

- -

66

99

107

< 20

138

103

< 10

92

128

< 20

142

147

-

19 Lindley

silt loam Des Moines, Iowa

1.1

3.7

5.7

7.6

9.7

11.6

0.4

1.4

2.1

2.6

2.6

3.1

- -

0.6

1.7

2.3

3.0

3.0

5.5

0.6

0.8

1.6

2.8

5.0

4.6

-

16

36

47

74

70

69

- -

29

70

104

159

118

207

< 10

70

100

177

176

259

-

21 Marshall

silt loam

Kansas City,

Missouri

1.5

4.0

6.0

1.6

2.5

4.4

- -

2.4

3.2

5.0

2.1

3.2

6.3

-

17

41

56

- -

< 10

71

101

< 10

53

57

-

30 Muscatine silt

loam Davenport, Iowa

1.1

3.6

5.7

8.2

11.6

17.0

1.2

1.6

3.0

4.8

9.3

8.2

- -

1.2

1.7

2.0

4.7

12.5

11.2

1.3

2.0

3.6

5.5

9.8

8.9

-

< 10

< 20

32

77

136

170

- -

< 10

< 20

25

49

143

140

< 10

< 20

34

79

117

344

-

44 Wabash

silt loam

Omaha,

Nebraska

1.1

3.6

5.7

7.6

11.6

1.7

1.0

1.0

1.6

3.8

- -

0.3

1.4

1.2

1.4

3.3

1.0

0.9

2.2

2.5

4.0

-

< 10

46

40

36

72

- -

< 10

81

50

44

65

30

59

37

53

69

-

46 Unidentified

silt loam

Denver,

Colorado

1.5

4.0

5.1

8.0

10.2

12.0

0.5

2.2

2.2

5.6

4.5

4.2

- -

2.5

4.0

2.3

5.3

4.0

5.6

1.8

5.5

3.6

6.6

8.1

8.1

-

15

< 20

26

50

54

68

- -

35

55

29

63

38

67

36

53

70

104

86

102

-

101

Billings

silt loam

(low alkali)

Grand Junction,

CO

1.9

4.1

9.3

- -

5.5

7.9

8.0

7.4

8.2

11.0

-

6.4

8.2

10.2

- -

76

103

165

49

128

203

-

41

61

128

102

Billings

silt loam

(moderate

alkali)

Grand Junction,

CO

1.9

4.1

9.3

- -

4.7

9.2

23.1

4.6

9.0

25.6

-

5.5

14.4

25.7

- -

63

99

247

54

90

293

-

57

130

410

103

Billings

silt loam

(high alkali)

Grand Junction,

CO

1.9

4.1

9.3

- -

6.3

28.1

45.2

14.1

14.6

42.4

-

13.7

42.8

58.6

- -

85

215+

215

99

132

361

-

96

208

418

35

2.6 Summary

Two backfills, concrete and aggregate, are considered for direct embedment. Concrete

encases the steel and help to protect against corrosion. This is a similar situation for other

reinforced concrete applications use with NDOT assets in corrosive soils. Aggregate backfill is

permeable and groundwater from the in-situ soil contacts the steel. This situation is of more

concern in corrosive environments. The typical backfill candidate is concrete and this is the best

choice in corrosive sites. Electric utilities use concrete encasement, galvanization, and a

protective mastic with success. A similar approach could be employed for the present work.

Table 2.10 provides a matrix of typical Nebraska soils and potential embedment options.

The culvert casing is not recommended due to compaction issues associated with the

corrugations at the soil-culvert. This was discussed previously in the constructability section.

Table 2.10: Tentative NDOT Applications for Direct Embedment Foundations

Foundation Type

Earth

Form

Concrete

Backfill

Earth

Form

Aggregate

Backfill

Permanent

Casing

Form

Concrete

Backfill

Permanent

Casing

Form

Aggregate

Backfill

Soil

Type

Loess

and Silt x x

Sand x x

Silty

Clay x x

Silt

36

3. HIGH MAST LOAD AND RESISTANCE

3.1 Introduction

This chapter provides an example of structural load calculations and the direct

embedment foundation resistance calculations based on a typical 120- and 140-ft HMTs

constructed in Nebraska. The structural loads were computed using the AASHTO LRFD

Specifications for Structural Supports for Highway Signs, Luminaires, and Traffic Signals

(2015) and a spreadsheet that was based on the fundamental principles of structural analysis

(strain compatibility, material properties, and equilibrium condition). The geotechnical

foundation resistance calculations were demonstrated through checking the vertical and

horizontal capacities. The horizontal load carrying capacity was calculated based on the

procedures introduced in the AASHTO LRFD Specifications for Structural Supports for

Highway Signs, Luminaires, and Traffic Signals (2015). This method of analysis was developed

by Broms (1964a and 1964b).

3.2 Structural Loads

3.2.1 General

The general drawings and the specifications of HMTs used by NDOT (Bushnell Tower

Project Plans and Specs, 2015). The pole was divided into twenty stations where static shear and

moments are computed assuming a uniform tape in diameter and thickness. Based upon the load

effects, the resulting curvature is integrated to calculate rotation, and the rotation is integrated to

calculate translation. The resulting translation is also used to compute the second-order load

effects (P-Delta effect).

The following example provides the steps to calculate the base reaction (axial, shear,

moment at the base) and tip displacement at the top of the tower for a 140-ft HMT with 12

37

luminaires which is the maximum possible for NDOT standards. The wind load in this example

is calculated following the AASHTO LRFD Specifications for Structural Supports for Highway

Signs, Luminaires, and Traffic Signals (2015). The Standard Specifications for Structural

Supports for Highway Signs, Luminaires, and Traffic Signals (2015) (allowable stress method)

was also used.

3.2.2 Dimensions and input parameters

The analysis was performed on a typical NDOT HMT make of steel galvanized tube that

tapers at a rate of 0.14in/ft vertically. This rate is typical and is set in the manufacturing process.

The pole is modeled with typical bending assumptions as outline above. The pole height above

grade is L = 140 ft, the top diameter is Dtop = 7.76 in., the bottom diameter is Dbot = 26.5 in., and

the thickness at the top and bottom is ttop = 0.1875 in. and tbot = 0.4375 in. The thickness

typically varies from section to section (three or four are typical). But, for numerical analysis,

the thickness was assumed to be varied at a constant rate which is more conservative regarding

the load increase and simple to implement. The luminaire has an effective projected area (12

luminaires) EPA = 18.72 ft2 weighing W = 3,450 lbs. NDOT specifications state 250 lbs per

luminaire, twelve luminaires, additional 15% added for arms and other assemblies.

3.2.3 Wind load

Wind load shall be based on the pressure of the wind acting horizontally on the HMT as

defined in Section 3.8 of the AASHTO LRFD Specifications for Structural Supports for

Highway Signs, Luminaires, and Traffic Signals (2015).

Design wind pressure

Pz = 0.00256 Kz Kd G V2 Cd (psf)

38

where:

V is the basic wind speed (mph),

Kz is the height and exposure factor defined Article 3.8.4,

Kd is the directionality factor defined in Article 3.8.5,

G is the gust effect factor defined in Article 3.8.6, and

Cd is the drag coefficient defined in Article 3.8.7.

Basic wind speed

The LRFD wind maps use an extreme wind matched with an ultimate strength

approach for resistance. AASHTO LRFD LTS 3.8-1b for a 700-year MRI (Mean

Recurrence Interval) Basic Wind Speed for Nebraska is 115 mph. However, the

basic wind speed is 114 mph for a 700-year MRI per the latest ASCE 7-16 which

is adopted within AASHTO LRFD LTS. Therefore, we used 114 mph as the basic

wind speed in this example.

Standard Specifications uses a wind speed of 90 mph with an allowable stress

approach for resistance. Both specifications were used to determine the load effect

at the top of the foundation (bottom of the pole). A brief commentary is provided:

AASHTO LRFD LTS 3.8-2b for a 1,700-year MRI (Mean Recurrence Interval)

Basic Wind Speed for Nebraska is 120 mph. The load and resistance factors are

calibrated to provide a reliability index of approximately 3.0 for 300-year MRI, 3.0

to 3.5 for 700-year MRI, and 3.5-4.0 for 1,700-year for main members. NCHRP

Report 796 provides the details of this calibration (Puckett, et al. 2014)

39

Based on the fact that the basic wind speed is squared in calculating the design wind

pressure, the basic wind speed used in this example will result in approximately

60% higher wind pressure than typical design calculations made following the

current NDOT standard specifications (e.g. (114/90)2=1.6); however the full

strength limit is used rather than only a fraction of the strength per allowable stress

design (standard specifications) The results should be essentially the same as the

standard specification. A secondary analysis was also performed with the Standard

Specification loads.

Height and Exposure Factor

The height and exposure factor shall be determined either from Table C3.8.4-1 or

calculated using the following equation.

Kz = 2(𝑧

𝑧𝑔)

2

∝

where:

z is the height above the ground at which the pressure is calculated, and

or 16 ft, whichever is greater

zg is a constant that varies with the exposure condition and based on ASCE/SEI

7-16, it should be taken to be 900 ft for Exposure C

is a constant that varies with the exposure condition and based on ASCE/SEI

7-16, it should be taken to be 9.5 for Exposure C.

40

Directionality Factor

The directionality factor is defined in Table 3.8.5-1 of AASHTO LRFD LTS

Article 3.8.5. The values in the table are consistent with those from ASCE 7-16 as based

upon work by Ellingwood (1981) and Ellingwood et al. (1982). Because the typical

drawings of the HMT used in Nebraska is a hexdecagonal (16-sides) shape, the

directionality factor is Kd = 0.95.

Gust Effect Factor

The gust effect factor, G, shall be taken as a minimum of 1.14. Information

presented in ASCE/SEI 7-16 states that if the fundamental frequency of a structure

is less than 1 Hz or if the ratio of the height to least horizontal dimension is greater

than 4, the structures should be designed as a wind-sensitive structure. Thus, the

structures being considered in AASHTO LRFD LTS are all under this category and

the gust effect factor shall be taken as a minimum of 1.14.

Drag Coefficient

The wind drag coefficient shall be determined from Table 3.8.7-1. According to

Table 3.8.7-1, poles that have hexdecagonal (16-sides) shape with design wind

speed higher than 78 mph, the drag coefficient, Cd, is 0.55.

3.2.4 Flexural response

The pole is divided into twenty sections assuming a uniform taper rate in diameter. The

calculated pole taper rate is (Dbot - Dtop)/L = 0.134 in./ft which is slightly lower than the typical

Valmont design with a 0.14 in./ft taper rate. Based on the input parameters and the pressure

calculated following the wind load parameters of the AASHTO LRFD LTS (2015), lateral load,

41

P, is calculated at twenty stations. This lateral load accumulated towards the base to calculate

the shear force, V. Next, shear force at each location multiplied by the length of each station

(total pole length divided by the number of stations) provides the moment, M, at each station.

Dividing the moment by stiffness, EI, will provide curvature, ∅ at each station. This is possible

based on the Bernoulli-Euler beam theory ‘plane section remains plane’, which gives the

connection between curvature and strain produced at each station. With the material properties

of steel, and the moment calculated at each station, strain can be calculated at each station and

dividing the maximum strain at each station by the ‘distance from the neutral axis to the location

strain is being calculated’ will provide curvature. In summary, using the statics (equilibrium

conditions; arithmetic), material properties (constitutive behavior; stress-strain relationship), and

the theory that plane section remains plane, the lateral load at each station was translated into

curvature. Finally, from geometry (compatibility), multiplying curvature with the length of each

station will provide rotation, 𝜃, at each station. By adding up all the rotations, slope from the

groundline can be calculated. In addition, rotation at each station multiplied by the distance

between each station to the groundline will provide translation (displacement), ∆ ,at each station.

By summing all translations, the tip translation at the top of the tower can be calculated.

As a final step, the axial load including the self-weight of the pole at each station and the

weight of the luminaire was multiplied with the translation at each station to take account the

second-order effects that increases the moment at each station. The load factor for dead load was

1.25 and for wind load 1.0. Using statics, material properties, and geometry, the three

fundamental principles for flexural analysis, the curvature, rotation, and translation was all re-

calculated considering the P-∆ effects. The total moment at the base increased by 12%

considering the second-order analysis. As a check, the moment magnifier calculated using the

42

simplified approach in AASHTO LRFD LTS Article 4.8.1 is 1.273 which is a higher value and

more conservative at the tower base. A typical moment diagram is illustrated in Figure 3.1

including the initial moment, second-order moment considering the P-∆ effects, and the

AASHTO LTS magnified moments using the magnifier calculated by the simplified approach.

The simplified approach provides higher moment below 60 ft but less moment above 60 ft than

the second-order moment calculated from the structural analysis.

Figure 3.1: Typical Moment Diagram from Analysis

43

3.2.5 Base reactions from structural analysis

The following

Table 3.1 summarizes the base reactions including the axial load, shear force, moment at

the base, and tip translation at the top of the 140-ft HMT. A typical moment diagram is

illustrated in Figure 3.1.

Table 3.1: Calculated Base Reactions for a typical Nebraska 140-ft HMT

AASHTO LRFD LTS Base Reactions (these are LRFD values)

Axial, kips 14.9

Adjust to ASD depending upon

foundation design method Shear, kips 4.8

Moment, ft-kips 382

Tip Translation, in 102

Base stresses, ksi 20.4 Assumes circular section

Base moment

magnification factor 1.12

3.3 Foundation Resistance for Nebraska High Mast Towers

3.3.1 General

Foundation resistance calculations were conducted to check the capacity of the pile

foundation for luminaire poles based on the combination of available information (e.g., structural

information) and unavailable (e.g., geotechnical information). For the unavailable information,

conservative engineering judgement was applied. In addition, the ultimate vertical and lateral

44

resistance of the pile foundation were computed and compared with the factored axial force,

shear, and moment.

3.3.2 Loads on foundation

Loading condition for this foundation is shown in Table 3.2. Primarily, the axial force

and moment from the structural analysis was used for checking the vertical capacity of the

foundation while shear and moment was used to check the horizontal capacity of the foundation

in this example.

Table 3.2: Calculated Base Reactions for a typical Nebraska 120-ft and 140-ft HMT

NDOT Foundation Load Effects

AASHTO LRFD LTS Base Reactions (these are LRFD values)

Pole Height, ft 140 120

Axial, kips 14.9 13.4 Adjust to ASD depending upon

foundation design method

Shear, kips 4.8 4.0

Moment, ft-kips 382 273

Tip Translation, in 102 53

Base stresses, ksi 20.4 14.6 Assumes circular section

Base moment magnification factor 1.12 1.09

Natural Period, T1, sect 5.0 4.0

3.3.3 Load carrying capacity

The vertical load carrying capacity of the HMT foundation was calculated based on the

following assumptions.

Foundation dimensions

Foundation diameter

45

D = 4 ft (assuming that the pole is embedded inside a concrete backfill, this

diameter will allow 1 ft of working space additional to the typical diameter of steel

poles used in Nebraska)

Foundation depth

L = 14 ft (10% of the height of the luminaire, a conservative assumption)

Net Load Carrying Capacity of a (single) pile:

Qu,net = Ap cu Nc*

where,

Ap is the cross-sectional area of the shaft. Ap = 𝜋𝐷2

4= 12.57 (ft2)

cu is the undrained cohesion (=200 psf, assumed highly saturated cohesive soil for

a conservative condition, Song et al., 2019) (ultimate strength)

Nc* is the bearing capacity factor (=6.5 when cu /Pa = 0.25, this magnitude of Nc

* is

practically a low limit from Das, 2014) (ultimate strength)

Pa is the atmospheric pressure cross-sectional area of a pile

Therefore, Qu,net = (12.57)(200)(6.5)/1000 = 16.3 (kips)

The factored axial load from the structural analysis was 14.9 (kips)

< Net load carrying capacity = 16.3 (kips)

This calculation is based on the assumption of using a closed end pipe pile. In addition,

the side friction of the embedded pole and the buoyance force are reviewed as follows:

Friction Capacity of a (single) pile:

Qf,net = As 𝛼 cu

46

where,

As is the surface area of the embedded pole, As = 𝜋𝐷𝐿 = 175.9 (ft2)

𝛼 is the reduction factor = 0.9 (could be 1.0 but used 0.9 to be conservative)

cu is the undrained cohesion (=200 psf, assumed highly saturated cohesive soil for

a conservative condition, Song et al., 2019) (ultimate strength)

Therefore, Qf,net =(175.9) (0.9) (200/1000) = 31.6 (kips). The total vertical resistance becomes

47.9 kips with the sum of bearing capacity and friction capacity.

Buoyancy Check:

B = Ap 𝛾𝑤 L

where,

Ap is the cross-sectional area of the shaft. Ap = 𝜋𝐷2

4= 12.57 (ft2)

𝛾𝑤 is unit weight of water (62.4 lb/ft3)

L is the embedded depth (14 ft in this example)

Therefore, B = (12.57) (62.4/1000) (14) = 11.0 (kips). Therefore, the buoyancy force is lower than

the summation of the net load bearing capacity (16.3 kips) and the side friction force (31.6 kips).

Since, there is sufficient side friction force that exceeds the factored axial load capacity and

buoyancy, respectively, a 2 in. hole at the base of the pole can be drilled on the bearing plate to

relieve the buoyancy force.

3.3.4 Horizontal load carrying capacity

The horizontal load carrying capacity was checked following the procedures introduced

on the AASHTO LRFD LTS Article 13.6.1 Commentary. The procedure outline a procedure to

47

compute the required shaft depth that provides sufficient lateral resistance for the given factored

moment and shear.

AASHTO LRFD LTS (2015) procedure

Required embedment length

The AASHTO LRFD LTS (2015) Article 13.6.1 provides the approximate procedures for

the estimation of embedment length in commentary. The method of analysis is based on

procedures developed by Broms (1964a and 1964b). For cohesive soil, the required embedment

length L can be determined as follows:

𝐿 = 1.5𝐷 + 𝑞 [1 + √2 +(4𝐻+6𝐷)

𝑞]

in which,

𝐻 =𝑀𝐹

𝑉𝐹, and

𝑞 = 𝑉𝐹

9𝑐𝐷 where,

D is the shaft diameter (ft)

c is the ultimate shear strength of cohesive soil (ksf)

MF is the factored moment at ground-line (kip-ft)

VF is the factored shear at ground-line (kip)

Based on the factored moment and factored shear provided from the structural analysis and

the assumed shaft diameter of 4 ft,

H = (382/4.8) = 79.6 ( eccentricity or equivalent height)

q = (4.8)/(9 x 0.2 x 4) = 0.67 (ksf)

48

Therefore, L = 1.5(4) + (0.67) x [1 + {2 + (4 x 79.6 + 6 x 4)/0.67}1/2] = 21.85 -> Use 22 ft

Maximum moment in the shaft

𝑀𝑢 = 𝑉𝐹(𝐻 + 1.5𝐷 + 0.5𝑞) = (4.8) x (79.6 + 1.5 x 4 + 0.5 x 0.67) = 412.5 (k-ft)

Location of maximum moment below ground-line

(1.5𝐷 + 𝑞) = (1.5 x 4 + 0.67) = 6.67 (ft) below ground-line

3.4 Summary

This foundation resistance calculation example demonstrates that the 4-ft diameter

foundation with an embedment of 14 ft (10% of the total height of the HMT) which was the

initial design values chosen may not satisfy the lateral resistance requirement for undrained

cohesive soil (200 psf) based on Brom’s graphical solution. Both the graphical solution and the

AASHTO LRFD LTS procedures demonstrated that the embedment length should be increase

approximately up to 17% of the total height (24 ft embedment length for the 140-ft pole).

However, this example is based on the conservative soil condition (reduced capacity) where

cohesive soils are used and with different soil conditions, the required embedment length can

change. Further numerical analysis with commercial software was conducted to perform

parametric study with varying soil conditions. The diameter can also be increased to satisfy the

strength required.

49

4. FOUNDATION SYSTEMS

4.1 Introduction

This chapter presents the results of the parametric study that was conducted using LPILE

and COMSOL with varying conditions. Based on the numerical analysis, various foundation

systems with different soil conditions were suggested for direct embedment options that NDOT

may choose for future projects. For the parametric studies, service level base moment and shear

from the HMT were used.

4.2 LPILE Analysis

4.2.1 Input parameters

A round concrete shaft with permanent casing and core/insert as shown in Figure 4.1 was

used to model the steel HMT directly embedded in soil with concrete backfill. The embedment

length of 24 ft was selected from the load and resistance calculations in the previous chapter.

The steel section, casing, and core/insert material properties can be selected to represent the pole

directly embedded in soil with concrete backfill. The casing outside diameter and casing wall

thickness were selected based on the outside diameter of the concrete backfill and thickness of

the concrete backfill (subtracting outside diameter with the average steel pole diameter),

respectively. The core diameter and core wall thickness values were selected based on the

average diameter of the steel pole, and average thickness of the steel pole between the diameter

and thickness at the ground-line and bottom of the pole at the 24 ft embedment base. The

outside diameter of the concrete backfill was 48 in., the thickness of the concrete backfill was 23

in., the average diameter of the steel pole used in this analysis was 24.9 in., and the average

thickness of the steel pole used was 0.4161 in. based on the properties received from the typical

cross sections.

50

The material properties of the steel section, casing, and core/insert is shown in Table 4.1.

The model was also prepared to not have concrete filled in the core for numerical analysis for the

second run. The first run of simulation was to test out the conditions of the actual site conditions

while the second run of simulation was conducted to check whether the stiffness of pole or

backfill material affects the behavior of different foundation systems.

Figure 4.1: Screen Capture of Figure 3.19 from LPILE Manual (Tab Sheet for Shaft

Dimensions of Drilled Shaft with Casing and Core to represent the Direct Embedment

Foundation)

51

Table 4.1: Input Material Properties for LPILE Analysis

Material Properties

First Run

(Round Concrete Shaft

with Steel Core)

Second Run

(Elastic Body: To check

how the soil LPILE

system behaves for

different modulus,

embedded depth, and

pile diameter)

Modulus of Steel Pole

(ksi) 29,000

First Trial: 36,000

Second Trial: 3,600 Modulus of Concrete

Backfill (ksi) 3,600

Cohesion of Soil (psf) 200 200

Yield Stress of Steel Pole

(ksi) 60 Elastic

Yield Stress of Concrete

Backfill (ksi) 4 Elastic

4.2.2 Behavior of lateral piles with round concrete shaft and steel casing

The service level unfactored load conditions applied in this analysis was 245 ft-kips

moment and 3.1 kips shear force at the pile head. For the 140-ft tall HMT with a 28 ft

embedment depth and 4 ft diameter clayey soil with 200 psf cohesive, the deflection computed at

the ground-line of the embedded foundation was 0.6 in. The deformed shape is linear as shown

in Figure 4.2 indicating that the pile behaved close to a rigid body. This also indicates that the

behavior of lateral pile is governed by the surrounding soil (soft clay with 200 psf cohesive).

Such behavior is known as the condition of “short pile”.

52

Figure 4.2: Deflection of the embedded steel pole based on LPILE analysis

The shear force and bending moment diagram from the LPILE analysis is shown in

Figure 4.3. The service level shear force and the bending moment applied at the pole head at the

ground-line can be observed in both plots. The change in shear force direction occurs at 16 ft

below the ground-line. The maximum bending moment is observed at 4 ft depth from the pile

head. There is approximately 3 ft difference from the maximum bending moment location

calculated using the AASHTO LRFD LTS procedures in the previous chapter. But the

calculations in the previous chapter were based on factored loads and the loads in the LPILE

analysis was based on service loads which will cause the difference. The bending moment

diagram on Figure 4.3 demonstrates the applied moment at the pile head and a gradual decrease

due to soil resistance, and zero bending moment at the pile base as expected for a “short pile”.

53

Figure 4.3: Shear Force and Bending Moment Profile for the embedded pole

The deflection of the pile head for different embedment length is plotted in Figure 4.4.

As shown in Figure 4.4, the top head deflection at the ground-line converges to less than 1 inch

when the pile length reaches 24 ft embedment length. This indicates that the selected 28 ft

embedment length will provide ground-line deflections less than 1 in.

Figure 4.4: Deflection of the embedded steel pole based on LPILE analysis

4.2.3 Effect of modulus of piles

As discussed in Table 4.1, in order to check the soil-pile system behavior for different

modulus, embedded depth, and pile diameter, a second set of numerical analyses was conducted

54

with material properties shown in Table 4.1. The input value varied from the modulus of 3,600

ksi to the 36,000 ksi. The left figure on Figure 4.5 is showing the deflection profile for the given

service load conditions for the lower modulus while the right figure on Figure 4.5 shows the

deflection profile for the higher modulus. As shown in Figure 4.5, there are negligible effect of

modulus on the behavior of these lateral piles for the 200 psf cohesive soil. Therefore, a

parametric study with a solid column for various diameters and embedment depth were

conducted.

Figure 4.5: Shear Force and Bending Moment Profile for the embedded pole

4.2.4 Parametric study for various foundation systems

For the parametric studies, the service loads (moment and shear) for the two typical

HMTs (140 ft and 120 ft) were considered. The horizontal displacement at the ground-line for

eight different embedment depth 28 ft, 26 ft, 24 ft, 22 ft, 20 ft, 18 ft, 16 ft, and 14 ft were

analyzed. Concrete backfill was used with five different types of natural soil surrounding the

backfill was studied: 200 psf cohesive clay, 400 psf cohesive clay, 600 psf cohesive clay, 30-

degree (friction angle) sand, and 35-degree sand. Two different backfill diameter was

55

considered: 4 ft and 3 ft. This backfill diameter was chosen because the typical diameter of the

140-ft HMT constructed in Nebraska is approximately 2 ft at the base and 6 in. to 1 ft working

space for concrete placement surrounding the 2 ft pole will result in 3 ft or 4 ft backfill. Table

4.2 summarizes the various foundation systems analyzed for the 140-ft HMT. The parametric

study started with a 28-ft embedment length which was the required length calculated based on

the AASHTO LRFD LTS procedures introduced in Chapter 3. For the worst condition with 200

psf cohesive clay surrounding the backfill, the ground-line deflection at the surface (at the head

of the pile) was 0.6 in. for 245 ft-kips moment and 3.1 kips shear (both surface level loads)

applied at the pile head. As the embedment length decreased to 24 ft, the ground-line deflection

was larger than twice the displacement shown for the 28-ft case (1.5 in. displacement for the 24

ft, and 0.6 in. for the 28 ft case). The analysis was conducted again for the case with 400 psf

cohesive clay. There the embedment length can be decreased to 22 ft from 28 ft with a relatively

better cohesion clay surrounding the backfill. Comparable ground-line deflection (0.55 in.) was

observed for the 22-ft embedment length pile surrounded by the 400 psf cohesive soil with the

case with 28-ft embedment when 200 psf clay is surrounding the backfill. The parametric study

was further extended to a 600 psf cohesive clay and the analysis demonstrates that the direct

embedment length can be decreased down to 16 ft to have comparable level of ground-line

deflection (0.7 in.) with the worst-case scenario (0.6 in. with 200 psf cohesive clay) as the

cohesion increased three times the worst case.

The parametric study was conducted for the 30-degree friction angle sand that showed

much less ground-line deflection for the 28-ft embedment length with the identical loading

conditions (approximately 1.33% of the deflection calculated for the 200 psf cohesive clay).

This is expected because sand typically have higher modulus than clay. With the 30-degree

56

sand, the embedment length can be decreased to 16 ft while still having less than half of the

ground-line deflection (0.24 in.) for the 28 ft embedment length with 200 psf cohesive clay (0.6

in.). The friction angle was increased up to 35-degree sand (stiffer sand) which demonstrated

that the deflection decreases to less than half of what was calculated for the identical embedment

of 16 ft with a 4-ft diameter backfill to 0.09 in. With such a small displacement, a smaller

diameter backfill was included to check the feasibility of having a smaller pile diameter. The

backfill was decreased from 4 ft to 3 ft. With the 16 ft embedment (this is 10% of the tower

height + 2 ft), the backfill diameter of 3 ft with a 35-degree sand had 0.14 in. ground-line

displacement. With the 3 ft backfill diameter, the study further investigated the ground-line

displacement with 30-degree sand and 200 psf cohesive clay. With the 30-degree sand, the 3-ft

backfill diameter pile with 16-ft embedment length resulted in 0.22 in. ground-line displacement

(approximately 1/3 of the worst-case scenario with 200 psf-clay, 4 ft backfill, and 28 ft

embedment length). One additional analysis was conducted for the 3-ft backfill with the 200 psf

clay with 24-ft embedment length, and the computed ground-line displacement was 2.2 in.

(approximately 50% more than the identical case with 4-ft diameter backfill).

The parametric study for a 120-ft tower was additionally conducted with the service level

moment of 177 ft-kips and 2.6 kips shear. The ground-line horizontal displacements calculated

from the analysis is shown in Table 4.3. With the loads decreased with a shorter tower height,

only the 3-ft backfill option was considered. With the worst case of 200 psf cohesive soil, 28-ft

embedment length was the only case that provided ground-line displacement less than 0.6 in.

However, as the cohesion was increased to 400 psf, the embedment was decreased to 20 ft

providing 0.55 in. ground-line displacement at the pile head.

57

Table 4.2: Parametric Study for Various Foundation Systems (140-ft Tower)

Case

(Pole Height – Embedment

Length – Pile Diameter -

Shear Strength – Soil Type)

Ground-Line Displacement

(in.)

140’-28’-4’-200 psf-clay 0.60

140’-26’-4’-200 psf-clay 1.10

140’-24’-4’-200 psf-clay 1.50

140’-28’-4’-400 psf-clay 0.18

140’-24’-4’-400 psf-clay 0.34

140’-22’-4’-400 psf-clay 0.55

140’-22’-4’-600 psf-clay 0.11

140’-20’-4’-600 psf-clay 0.20

140’-18’-4’-600 psf-clay 0.28

140’-16’-4’-600 psf-clay 0.70

140’-28’-4’-30 degree-sand 0.08

140’-24’-4’-30 degree-sand 0.11

140’-22’-4’-30 degree-sand 0.12

140’-20’-4’-30 degree-sand 0.15

140’-18’-4’-30 degree-sand 0.18

140’-16’-4’-30 degree-sand 0.24

140’-16’-4’-35 degree-sand 0.09

140’-16’-3’-35 degree-sand 0.14

140’-28’-3’-30 degree-sand 0.16

140’-24’-3’-30 degree-sand 0.16

140’-20’-3’-30 degree-sand 0.20

140’-16’-3’-30 degree-sand 0.22

140’-24’-3’-200 psf-clay 2.20

58

Table 4.3: Parametric Study for Various Foundation Systems (120-ft Tower)

Case

(Pole Height – Embedment

Length – Pile Diameter -

Shear Strength – Soil Type)

Ground-Line Displacement

(in.)

120’-28’-3’-200 psf-clay 0.45

120’-24’-3’-200 psf-clay 1.00

120’-22’-3’-200 psf-clay 2.50

120’-28’-3’-400 psf-clay 0.13

120’-24’-3’-400 psf-clay 0.21

120’-22’-3’-400 psf-clay 0.26

120’-20’-3’-400 psf-clay 0.55

120’-20’-3’-600 psf-clay 0.14

120’-18’-3’-600 psf-clay 0.20

120’-16’-3’-600 psf-clay 0.41

120’-14’-3’-600 psf-clay 0.90

120’-24’-3’-30 degree-sand 0.09

120’-20’-3’-30 degree-sand 0.13

120’-16’-3’-30 degree-sand 0.16

120’-14’-3’-30 degree-sand 0.22

120’-14’-3’-35 degree-sand < 0.22

With the 600 psf cohesive soil, it was found that 10% of the tower height + 2 ft with a 3

ft diameter backfill will still provide ground-line displacement less than 0.5 in. With 30-degree

or 35-degree sand the 120-ft HMT with 3-ft diameter backfill with an embedment length of 10%

of the tower height will only produce approximately half of the displacement (0.22 in. or less)

observed in the worst-case scenario with 200 psf cohesive clay as the surrounding soil.

59

4.3 COMSOL Analysis

4.3.1 Input parameters

Geometry and Loads

Because of the sensitive nature of soil-structure interaction and the possibility of

encountering weak clay soil in eastern Nebraska, a finite element model was developed as an

independent check on the LPILE and Brom’s analysis.

In this analysis, the steel pole was modeled as tube that has the height above grade equal

to the eccentricity associated with the service load effects, Mbase = 245 ft-kips and Vbase = 3.1

kips. The equivalent load effects are Ptip = 3.1 kips applied at Lbeam = 245/3.1 = 79 ft.

The pole is surrounded by a “backfill” that is either concrete or large aggregate, see the

green domain in Figure 4.6, which is surrounded by soil that is assumed to be either a soft

cohesive clay or sand (cohesionless).

Figure 4.6: Domain Geometry

60

The base geometry properties are provided in Table 4.4. Figure 4.7 provides a close up

of the tube, backfill, and soil interfaces. Figure 4.8 provide the detail of the bottom of the

domains. Note that the pole and the backfill do not extend to the bottom of the soil domain

thereby permitting movement. The soil domain base boundary condition is considered fixed;

however, the depth below the pole/fill is sufficiently large to obviate boundary effects associated

with stiffening the pole’s base boundary. This is also illustrated in elevation in Figure 4.9. The

pole was modeled with solid elements, this was a matter of convenience as its role is to: provide

proper load effect to the fill and provide proper stiffness within the fill. Note that the fill,

whether modeled as aggregate or concrete is significantly stiffer than the surrounding soil. This

is illustrated below in the analysis results that show near rigid body behavior.

Table 4.4: Geometric Properties for COMSOL model

Geometric

Dimension Value Comment

r1 12[in] inside tube radius

thickness 0.5 [in] tube thickness

r2 r1+thickness outside tube radius

r3 24[in] fill radius (concrete or aggregate)

r4 5*r3 soil radius, large to avoid

boundary effects

PoleBottom 2[ft]

distance of bottom to model

boundary, large to avoid boundary

effects

h2 24[ft] Foundation depth (model depth)

h1 h2-

PoleBottom Pole depth

r5 r4+1[ft] Outer radius for infinite model

h3 Lbeam Pole ht above fill

61

Figure 4.7: Close Up of Tube, Fill, and Soil Domains

Figure 4.8: Bottom of Domain

Figure 4.9: Elevation

62

Material Properties

The material properties are provided in Table 4.5. The clay properties are listed first and

the EsoilSoftClay parameter was used in the analysis. Sand, concrete, and steel properties are

listed, respectively.

Table 4.5: COMSOL Material Properties

Property (variable) Value Typical Range

Clay

CohensionClay (200/144) [psi] 12-384 kPa

EsoilClay 20[MPa] 20-200 MPa

InternalFrictionClay 10[deg]

DilitationAngleClay SmallNumberDeg[deg] make small

number

UniaxialCompressionClay CohensionClay

UniaxialTensionClay CohensionClay

EsoilSoftClay 3 [MPa] 1-3MPa

Sand

EsoilSand 10[MPa] 10-50 MPa

InternalFrictionSand 30[deg] 30-40 deg

CohensionSand 1 [psi]

TensionMaxSand CohensionSand

Concrete

ConcreteCompressionStrength 4000[psi]

ConcreteTensileStrength ConcreteCompressionStrength/10

ConcreteGamma 145[lbf/ft^3]/g_const

Econcrete 3600000[psi]

Steel

SteelGamma 490[lbf/ft^3]/g_const

Esteel 29000000[psi]

4.3.2 Analysis (Clay Soil)

First, the results from the finite element model for a soft clay is presented. This is likely

the most critical application for the embedment design. The sandy soil is presented thereafter.

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Figure 4.10 illustrates the tip-loaded pole and the von Mises stresses in the pole. These

stresses are of little consequence in this analysis as the pole is designed and checked using the

AASHTO Specification. However, the bending behavior certainly is as expected. Additionally,

statics checks were performed to ensure that the applied load at the tip is appropriately balanced

by the reactions of the soil domain; this included horizontal forces and overturning moment.

Note the load factor illustrated in the following plots is Lfactor = 1.8 times the stresses due to the

expected service loads, e.g., Figure 4.10.

Figure 4.10: Pole Von Mises Stresses (Clay)

The displacement field is shown in Figure 4.11. Note the top of the pole moves in the

direction of the load and the bottom of the pole moves in the opposite direction. Both

translations are distributed vertically along the pole and decrease to a center of rotation. See

Figure 4.12. Note that the translation line is linear, therefore the pole and fill act as a nearly rigid

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body as compared to the deformation of the soil. This is as expected and consistent with LPILE

and Brom’s analyses. The typical stress field for normal stresses is shown in Figure 4.13.

Figure 4.14 provides a typical load vs. translation plot. Note that some nonlinearity exists

indicating that the soil is experiencing plastic behavior. Figure 4.15 illustrates a Boolean plot

where red indicates locations plastic strains exist which are primarily near the top of the fill and

near the bottom of the file. These areas create a force couple that resist the overturning moment.

The magnitudes, however, are small as shown in Figure 4.16.

Figure 4.11: Translation in the direction of load

65

Figure 4.12: Translation along the fill edge (Clay)

Figure 4.13: Normal Stresses in the direction of loading (Lfactor = 1, Clay)

66

Figure 4.14: Load-translation plot (Clay)

Figure 4.15: Boolean plot where plastic strains exist (red = yes, blue = no)

67

Figure 4.16: Effective Plastic Strain

The simulation was rerun at Lfactor = 1 several times, while changing the soil modulus

of elasticity, Esoil over a range to values. The translation profile is plotted for each analysis. As

expected higher Esoil the smaller the displacements. Note that as the soil become stiffer relative

to the tube and fill, a slight nonlinear translation, i.e., flexing of the pole/fill is illustrated. In all

cases, the center of rotation is approximately 150 in = 12.5 ft from the top of grade, or about

0.625 of the shaft depth.

The simulation was rerun by changing the distance from the bottom of the domain to the

bottom of the fill, i.e., this decreases that shaft depth. The results are shown in Figure 4.17 and

Figure 4.18. As expected the load effects in the soil increase with a shorter shaft; however,

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results are promising that both shaft depth and diameter could be optimized in the future based

upon a refined analysis in Comsol and/or LPILE analysis.

Figure 4.17: Parametric Study Esoil (Lfactor = 1, Clay)

Figure 4.18: Parametric Variation Shaft Depth = 28-ft distance from bottom (Lfactor = 1,

Clay)

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Figure 4.19: Pole Bottom vs Effective Plastic Strain (Shaft Depth = 24 ft – Pole Bottom,

Clay)

4.3.3 Analysis (Sandy soil)

The model was changed to a sandy soil with other properties remaining the same. Figure

4.20, Figure 4.21, Figure 4.22 illustrate the results. There were small and localize plastic strains.

Figure 4.20: Load vs Max Translation (sandy soil)

70

Figure 4.21: Translation in sandy soil

Figure 4.22: Translation along fill for sandy soil

71

An independent finite element model of the direct-embedment foundation was developed

and documented. Clay and sandy soil were modeled. The results indicate reasonable

performance with load effect less than the results from the LPILE model. This COMSOL

simulation may be used in the future for more detailed soil model and optimized design, perhaps

to decrease the shaft diameter and/or embedment depths. Field testing should be performed to

validate this level of refined modeling. The LPILE results are larger and are used below.

4.4 Selection Matrix based on Numerical Analysis

The recommended direct embedment length for NDOT was selected based on the criteria

of having horizontal ground-line displacement less than 0.6 in. for clayey soils and 0.3 in. for

sandy soils. The final selection matrix for various direct embedment foundations are provided in

Table 4.6.

Table 4.6: Selection Matrix for Various Direct Embedment Foundations

Pole Height (ft)

Direct

Embedment

Length (ft)

Diameter of

Backfill (ft) Soil Type

Expected

Ground-line

Displacement (in.)

Service Loads

140

28 4 200 psf clay 0.6

M =

245 ft-kips

V = 3.1 kips

22 4 400 psf clay 0.55

18 4 600 psf clay 0.28

16 4 30-degree sand 0.24



120

28 3 200 psf clay 0.45

M =

177 ft-kips

V = 2.6 kips

20 3 400 psf clay 0.55

16 3 600 psf clay 0.41


14 3 35-degree sand < 0.22

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5. SITE CONSIDERATIONS AND CONSTRUCABILITY

5.1 Introduction

This chapter provides information related to possible issues NDOT may encounter during

the construction of direct embedment foundations for High Mast Towers. Based on the

construction issues listed in this chapter, a draft construction specification is listed in Appendix

A as a reference to potentially mitigate the construction issues introduced in this chapter. In

addition to the constructability, cost comparison of selected foundation systems, local soil

conditions in Nebraska, and site considerations and steps for corrosion protection strategies that

can be implemented in Nebraska is discussed in this chapter.

5.2 Construction Issues

5.2.1 Overview

As outlined previously, drilled shafts are common foundations for high-mast poles and

other ancillary structures such as signs, traffic signals, and so forth. Local contractors likely

have this experience and are familiar with the geotechnical challenges. Hole stability is

paramount for both standard foundations and direct embedment. Stability is a concern for both

safety and the proper consolidation of backfill; stability can be addressed in several different

ways depending upon the types of soils, layering, and the moisture condition.

Geotechnical report attempts to define the local conditions of the soils to inform both the

designer about the size and foundation type; but most importantly, to advise constructor about

the site conditions regarding the construction method. However, geotechnical information may

be limited for a particular site, and in fact, reports might be based upon previous bores located

away from the site.

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The excavation must be closely observed and the contractor must be ready to react if the

soil is not representative of the report. The typical methods are outlined next. This discussion is

general and specific requirements are outlined in the draft specification provided in Appendix A.

5.2.2 Earth formed

The soil is stable and supports the integrity of the shaft diameter. This is the most

desirable situation. Unfortunately, it is so desirable that many contractors are overly optimistic

and push this type of construction in poor soil conditions. Difficulties arise when the soil starts

to slough and create voids in the walls. These voids are very difficult to fill and compact

properly, more so if the backfill material is aggregate. Another concern is that sloughing occurs

as the pole is being set and the additional soil in the bottom of the shaft affects the embedment

depth. In addition, the pole may bear at the bottom of the shaft against the sloughed soil rather

than the competent backfill material creating possible structural integrity concern.

5.2.3 Polymer slurry

Slurries fill the excavation with dense fluid-like material that can help to maintain the

shaft integrity. This method can be used for poor soils with the exception of gravel. Polymer

slurries are used frequently in the electrical utility industry, but placement can be tricky:

The shaft drill cannot be observed, so no way to directly detect a problem.

Indirect observations become important such as observing depth constantly to ensure no

sloughing is occurring.

Backfill placement is critical and typically concrete is used. The concrete displaces the

slurry as the concrete is placed from the bottom of the shaft upward. A tremie or concrete pump

74

is required. Aggregates are not recommended because of separation of fines and difficulty with

compaction.

5.2.4 Culvert

Using pipe culvert is a common method for hole stability because the material is

relatively cheap and readily available. However, corrugated ribs in the culvert provide areas

where the interface between the culvert and soil cannot be properly compacted creating voids.

These voids should be filled with grout to insure the integrity of the bearing surface, which is

important, especially for high-mast applications where the overturning moments are high

relatively to the downward thrusts. Grouting can be expensive.

5.2.5 Permanent casing

Casings are commonplace and offer one of the best methods to insure shaft stability.

Casings use, however, is the most expensive method of all outlined here. With this method the

contractor advances the casing down the hole as drilling progresses. There is little chance of

sloughing as the drill picks up any materials between the hole bottom and the bottom of the

casings. Once the casing has reached required shaft depth, the pole can be set.

5.2.6 Setting the pole

Setting the pole for direct embedment is a critical operation. The lateral support of the

pole must be of sufficient control and stiffness to maintain the pole’s position during backfill that

will include concrete pumping and potential wind challenges. The design will provide a section

of pole above grade and this section must be checked for adequacy and consistent with the

planned equipment. Adjustment after backfill placement is difficult, and certainly impossible,

after concrete hardening. The specifications call for a maximum deviation of plumbness. Unlike

the anchor-bolt methods, there are no leveling nuts to adjust for plumbness. Finally, poles has

75

protective coating of galvanizing and/or mastic. Proper handling is necessary to not scrape or

otherwise damage these coatings. Damage must be repaired by hand by application of coatings

prior to backfill. Small uncoated areas can lead to corrosion “hot spots” that can affect the

integrity of the system over time.

5.2.7 Backfill

The backfill process is critical. The design must allow for enough space between the pole

and the soil for the backfill operation to occur. This process must be monitored to make sure that

the consolidation or compaction (depending on the backfill) is to specifications. If the hole is

earth formed the contractor needs to take the necessary precautions to not cause sloughing into

the placed backfill material. The pole also needs monitored so that there is minimal

displacement during the backfill placement and the plumbness tolerances are maintained. On

site owner inspection should occur during the backfill operation, once fill is placed there is no

way to discern that critical problems did, or did not occur.

5.3 Locality of Soil Conditions in Nebraska

The highly-populated area of Nebraska – where Lincoln and Omaha is located (the area

within the red boundary in Figure 5.1), the shales are bed rocks covered by glacial tills and

intermittent Loess layers/pockets as summarized in Pabian (1970). These layers are the local soil

conditions for major structures such as cut/fill slopes, structural foundations, subgrade for

highways, dams and levees.

The weathered shales contain approximately 50% fine contents. Swelling pressure of

these weathered shales is in the range of 10 kPa to 24 kPa according to Song et al. (2019). The

24 kPa swelling pressure is high enough to nullify the effect of overburden thickness up to 40 ft.

This is high enough to open cracks in these soils. In addition, the temperature condition in this

76

area is frequently below negative 4 degrees Fahrenheit (-20 ℃) during winter days, but it is

frequently above 104 degrees Fahrenheit (40 ℃) during the summer days providing a

temperature fluctuation of 108 degrees Fahrenheit. Freeze-thaw cycles combined with wet-dry

cycles may cause substantial strength reduction for these soils.

Figure 5.1: Geologic Bed Rock Map of Nebraska (retrieved from Pabian, 1970)

Glacial tills typically contain 10% to 25% fines with occasional inclusion of core stones,

and their strength characteristic is significantly affected by the water content. The glacial tills at

77

the surface during the rainy season show immeasurably low vane shear strength while it is in the

range of 100 t/m2 during dry seasons (Song et al. 2019).

Back calculated strength of failed slopes in Nebraska by Song et al. (2019) showed that

the field strength of these soils at failure could be in the range of 70% to 10% of the initial

strength. Extensive laboratory test results by Stark and Eid (1994), Stark et al. (2005), and

Wright et al. (2007) also demonstrated that substantial strength degradation for highly plastic

soils are observed through ring shear tests and drained triaxial compression tests. Findings of

these previous studies indicate that weathering induced strength degradation of surface soils in

this area could be an underlying source of troubles for many different geotechnical structures.

Therefore, in the eastern part of Nebraska, a critical shear strength (200 psf for clayey

soils, internal friction angle of 30 degrees for sandy soils) is recommended to be used in

designing direct embedment foundations for HMT. In other areas, field testing such as field

vane or CPT tests are recommended to obtain the correct soils parameters, and corresponding

foundation embedment depth similar to what is listed in Table 4.6. For the case with no

available direct field test results of clay cohesion or internal friction angle for sand, NDOT

engineers are recommended to resort to SPT vs. cohesion, and/or SPT vs. internal friction angle

relationship available in Appendix B.

When there is absolutely no information regarding the soil conditions, particularly areas

outside Eastern Nebraska, it is recommended to use a cohesion of 200 psf for clayey soils,

internal friction angle of 20 degrees for sandy soils and use the Broms’ method to compute the

required embedded depth conservatively. When LPILE is used, special care must be taken

because the stiffness calculation module in LPILE may not properly accommodate this internal

78

friction angle lower than 29 degrees, though it may conduct the calculation and present an output

file.

5.5 Site Considerations and Steps for Corrosion Protection Strategies

Although the NIST field data introduced in Chapter 2 is an extensive field data that can

be used for quantitative design, it may be less practical to conduct corrosion design based on this

data. It may make more sense to conduct the preliminary design with a qualitative method

similar to the DIPRA design decision tables. The likelihood, consequences, and the possible

protection method will determine the initial design for corrosion resistance. The field data

produced by NIST can be used as a reference to what possible pit depths could be estimated and

the service life of the structure that will be buried under soil. The followings are suggested

procedures for corrosion protection strategies in Nebraska based on the literature review:

1. Check with the NDOT M&R Division whether the site is in a corrosive area. The

definition of corrosion area could be similar to what CALTRANS is using for their

judgement. For example, the site is in a corrosive area if chloride concentration is 500

ppm or greater, sulfate concentration is 2,000 ppm or greater, or the pH is 5.5 or less.

2. If the site is in the corrosive area, and the soil resistivity is equal or less than 1,000 ohm-

cm (both AWAA and CALTRANS classify this to be a highly corrosive area), the degree

of aeration (oxygen concentration), amount of moisture, pH level (hydrogen activity of

the solution) and ion contents or organic contents should be additionally measured by the

corrosion consultant or expert hired by the NDOT M&R division to collect additional

information that will affect the corrosion rate of the embedded HMT.

79

3. Depending on the HMT embedment length, pole diameter, and access availability for

repair work (if necessary), and the likelihood of underground corrosion (based on the

parameters measured in step 2), the NDOT M&R division may make decisions which

corrosion protection method to choose. Available methods include protective coatings

with epoxy or bituminous material, cathodic protection, increasing the pole thickness

below the ground-line, or use reinforced concrete jackets around the steel pole.

4. If both the likelihood and the consequence scores are calculated to be high, a combination

of the corrosion protection method listed in step 3 can be applied.

5. If necessary, the NIST tables provided (old reference but the most extensive field data up

to date) can be used to compute the potential maximum pit depth and weight loss of the

embedded pole. This will provide the expected service life of the embedded HMT

structure. Based on the expected life calculations, the design can change as needed.

5.6 Cost Comparisons

This section looks into the cost comparisons made for the conventional High Mast Tower

with the bolted base plate versus the case with direct embedment. The goal of the project is to

eliminate the fatigue-prone pole to base detailing by directly embedding the High Mast Tower

and improving the performance not necessarily to save the cost. However, it is anticipated that

there will be some cost difference due to the change. For example, since the baseplate to tube

connection is eliminated, and there is no need for anchor bolts, the fabrication cost will most

likely decrease with the change in detailing. The handhold does not change for both cases and

the slip connection for the remaining sections are based on standard practice for both cases

which will not likely to change the cost. Table 5.1 summarizes the estimated cost difference

including the material and fabrication cost of the pole, concrete and rebar required in both

80

construction methods, and anchor bolts needed in the conventional construction. It is anticipated

that there will be approximately $9,278 cost savings per pole.

Table 5.1: Cost Comparisons between Conventional vs. Direct Embedment (USD)

Bolted with Base Plate Direct Embedment Savings with Direct

Embedment

Pole Difference $2,458 $0 $2,458

Concrete and Rebar $2,300 $480 $1,820

Anchor Bolts $5,000 $0 $5,000

Summary $9,758 $480 $9,278

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6. SUMMARY AND CONCLUSION

6.1 Summary

High-Mast Tower (HMT) foundations have been traditionally designed and constructed

using cast-in-place foundation with anchor bolts that are used to secure the tower to the

foundation. This type of design requires a base plate that is welded to the tower shaft. The

Nebraska Department of Transportation (NDOT) has recently experienced issues with stresses

that this type of design presents at the anchor bolt/foundation or base plate/tower shaft interface.

This research project objective was to develop an alternative design for HMT foundations with

direct embedment of the HMT which can eliminate fatigue-prone details associated with the

pole-to-base plate connection which is the primary location of failure.

First, the literature that includes research from academia and industry, current and

proposed state of practice from industry, examples of design specifications and guidelines, and

corrosion for buried structures were reviewed. Secondly, structural loads for the typical 120- and

140-ft HMTs constructed in Nebraska and the soil resistance for them were calculated. The

structural loads were computed using the AASHTO LRFD Specifications for Structural Supports

for Highway Signs, Luminaires, and Traffic Signals (2015), with a spreadsheet based on the

fundamental principles of structural analysis. The geotechnical foundation resistance

calculations were made to check the vertical and horizontal soil capacity for the typical HMTs

used in Nebraska. Further parametric study was conducted using two numerical software:

LPILE and COMSOL for varying soil conditions and foundation systems with different

embedment length and backfill diameter for the service level base moment and shear. Required

embedment length and backfill diameter is provided as a matrix using the LPILE analysis results

which provided higher values than the finite element model created with COMSOL. Finally,

82

based on the site considerations and construability, a draft design specification and soil

parameters that can be used for Nebraska soil conditions are provided with details listed in the

Appendix of this report.

6.2 Conclusion

Based on the literature review, concrete and aggregate are recommended as backfill for

direct embedment of HMT. The typical backfill candidate is concrete since this is the best

choice in corrosive sites. Earth form or permanent casing form with concrete or aggregate

backfill is recommended for Nebraska soil. Culvert casings are not recommended due to

compaction issues associated with the corrugations at the soil-culvert interface.

From the load and resistance calculations, the shear force at the base (ground-line) for the

120-ft and 140-ft HMT were 4.0 kips, and 4.8 kips, respectively. The moment at the base for the

120-ft and 140-ft HMT were 273 ft-kips, and 382 ft-kips, respectively. The axial loads at the

base were 13.4 kips for the 120-ft HMT and 14.9 kips for the 140-ft HMT. Without considering

the side friction of the embedded pole, the net load carrying capacity of the HMT is sufficient to

resist the axial loads subjected at the ground-line. The required embedment length for the worst-

case soil conditions (200 psf cohesive clay) based on the AASHTO LRFD LTS procedures was

24 ft for the 140-ft pole.

Several foundation systems were recommended as a result of the parametric study

conducted by LPILE for different soil conditions. In order to maintain a ground-line

displacement of 0.6-inch, 28 ft embedment with a 4 ft diameter backfill will be required for a

140-ft HMT embedded in 200 psf cohesive clay. This embedment length can be decreased down

to 18 ft (10% pole height plus 4 ft) as the cohesion increases up to 600 psf. If the natural soil

surrounding the directly embedded HMT is sand with an internal friction angle of 30 degrees, the

83

embedment length can further be decreased to 16 ft which is 10% of the pole height plus 2 ft.

The backfill diameter can further be decreased to 3 ft from 4 ft if the internal friction angle is

increased 5 degrees. The 120-ft HMT embedded in 200 psf cohesive clay may require 28 ft

embedment length with a 3 ft diameter backfill to maintain ground-line displacement below 0.6

in. The embedment length can be decreased to 16 ft (10% pole height plus 4 ft) if the cohesion is

increased three times the worst-case scenario. For the 120-ft HMT, with 30-degree internal

friction angle sand, the required embedment can decrease down to 14 ft which is 10% of the

tower height plus 2 ft when, 0.3 in. is the target ground-line displacement.

Finally, based on the construction issues listed for earth formed, polymer slurry, culverts,

and permanent casings, earth formed and permanent casing were the two options recommended

for Nebraska HMTs. Appendix A provides specifications based on the two critical operation in

construction which is setting the pole and installing backfill. Based on the locality of soil

conditions in Nebraska, if Standard Penetration Testing data are available, methods introduced in

Appendix B can be used to calculate the soil parameters required in the design process. If soil

conditions are not available, it is recommended to use a cohesion of 200 psf for clayey soils and

an internal friction angle of 20 degrees for sandy soils and use the Brom’s method to compute

the required embedded depth conservatively. Since, the corrosion activity for buried structures

can be affected by many different scenarios, a five-step procedure for corrosion protection

strategy is provided in Section 5.5 of this report. The first step includes evaluating the site if it is

a corrosive area or not based on the threshold values of chloride concentration with 500 ppm or

greater, sulfate concentration of 2,000 ppm or greater, and the pH being 5.5 or less. If the site is

in the corrosive area, and the soil resistivity is equal or less than 1,000 ohm-cm, further site

measurements should be taken including the amount of moisture, pH level, ion contents, and

84

organic contents to characterize the site soil conditions. Based on these measurements the

NDOT Materials and Research division should choose either to use protective coatings with

epoxy or bituminous material, cathodic protection, or increase the pole thickness (yet, with the

concrete backfill, this may not be required). If the likelihood and the consequence of corrosion is

specifically high in the site where the HMT will be constructed, a combination of the corrosion

protection methods listed above can be applied. If the corrosion rate (pit depth per years) is

required to calculate the expected life, or the required increase in thickness of the pole buried

underground is needed, the NIST tables provided in Chapter 2 can be utilized as a reference. It is

anticipated that there will be a cost saving of approximately $9,300 per pole with the reduced

baseplate detailing with 6 to 8 anchor bolts required in conventional construction additional to

the reduction in reinforced concrete required for the foundation.

6.3 Further Research

An independent finite element model of the direct-embedment foundation using

COMSOL was developed and documented in Chapter 4. The results indicate reasonable

performance with load effects less than the results from the LPILE model. This COMSOL

simulation can be used in the future for more detailed soil model and optimized design to

decrease the shaft diameter and/or embedment depths. In order to have this level of refined

modeling, additional field testing should be conducted.

In addition, since the calculations provided in this report is based on assumptions for the

soil parameters, the research team can further engage with the NDOT Materials and Research

division to conduct field demonstration including design and construction of a full-scale HMT

with different site conditions.

85

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https://www.nrcs.usda.gov/wps/portal/nrcs/main/ne/soils/

Wong, D.O. (1991), “Subgrade Modulus of Laterally Loaded Piles in Clays,” Transportation

Research Record No. 1331, pp.67-78.

Wright, S.G., Zornberg, J.G. and Aguettant, J.E. (2007), The Fully Softened Shear Strength of

High Plasticity Clays, FHWA/TX-07/0-5202-3, 132p.

https://www.nrcs.usda.gov/wps/portal/nrcs/main/ne/soils/

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APPENDIX A

NDOT Sample Construction Specification for Direct Embedded Poles

Qualifications and Submittals

Submit the following for review at least 10 working days before constructing drilled

piers. NDOT review of the Contractor’s personnel qualifications and installation

plan does not relieve the Contractor of the responsibility for obtaining the required

results in the completed work.

Personnel Qualifications

Construction Personnel. Use a supervisor with at least three years of experience

in the construction of direct embedded poles. Supervisor must remain on-site

during all direct embedment installation activities. Upon request provide a resume

of job experience, project description, the owning agency’s name, email address,

and phone number.

Submittals

Furnish the following in the installation plan:

A. Details of proposed pier drilling methods; methods for removing materials

from the piers; procedures for maintaining correct horizontal and vertical

alignment of the excavation; and a disposal plan for the excavated material.

B. A description, including capacities, of the proposed equipment to be used

including cranes, drills, drilling unit, augers, bailing buckets, and final

cleaning equipment.

C. Demonstrate an understanding of the subsurface conditions at the site.

Reference the available geotechnical report and/or any other subsurface data

provided by the Company.

D. Details of methods to be used to ensure drilled pier hole stability during

excavation and concrete placement. Include a review of the chosen method’s

suitability for the anticipated site and subsurface conditions. If permanent

casings are proposed or required, provide casing dimensions and detailed

procedures for permanent casing installation.

E. As applicable, details of bracing, centering centralizers, and lifting and

support methods.

F. Details of Aggregate placement including compaction methods.

G. Details of concrete placement including proposed operations procedures for

free fall, tremie, or pumping methods. Provide summary of proposed actions

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to be taken when concrete does not meet minimum specifications or when

unforeseen delays occur during the concreting process.

Other Required Submittals

1. Concrete Mix Design

2. Aggregate gradation

3. Direct Embedment Installation Record

Execution

Drilling Operations

A. Excavate holes according to the installation plan. Report all deviations from

the plan to the onsite inspector.

B. When required, casings shall be installed as the drilling proceeds or

immediately after the equipment is withdrawn to prevent sloughing and

caving of the excavation walls. Casing shall be advanced ahead of the

drilling operation in order to maintain a soil plug capable of producing a

positive seal at the bottom that prevents piping of water or other material

into or out of the hole.

C. Slurry may be used to stabilize the excavation; however, a specific plan,

including the material to be used, must be submitted to NDOT for review

prior to use. Refer to FHWA Standard Specifications for Construction of

Road and Bridges, Section 565 “Drilled Shaft Installation” for all slurry use

requirements.

D. Steel casings of ample strength to withstand handling and installation

stresses shall be used. Use casing with the outside diameter equal to or

greater than the specified diameter of the pole and the inside diameter not

exceeding the specified diameter of the pole by more than 6 inches.

E. Each drilled shaft shall be accurately located, sized and plumbed. The

maximum deviation of the drilled pier from its designated location shall not

be more than 2 inches at its top elevation. The drilled shaft shall not be out

of plumb more than 1 inch in 5 feet of height.

F. Each drilled excavation shall be made to the approximate depth indicated on

the drawings. All weathered and loose material shall be removed from the

excavations. NDOT shall verify the final tip elevation before concrete or

aggregate placement. Classification of the excavated materials will not be

made except for identification purposes. Drilled excavation shall include

the removal and handling of all excavated materials from the site.

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G. All drilled excavations will be inspected by NDOT before the placement of

concrete or aggregate. All drilled excavations that cannot be visually

inspected shall be treated as a wet hole. Refer to wet method for concrete

placement.

Aggregate Placement

A. Backfill: Holes shall be backfilled with crushed aggregate backfill as specified

on the Drawings.

B. Backfill shall be compacted until the material has reached the required

compaction

C. Native and engineered backfill shall be banked and tamped twelve (12) inches

above the natural ground surface.

D. Surplus excavated material shall be evenly spread along the right-of-way or

hauled to an offsite location for dumping, according to the permissions and

requirements of each individual landowner.

E. Lifts of backfill material shall not exceed six inches in depth. Any extremely

dry materials shall be dampened during the backfill operation to obtain the

desired density

Concrete Placement

A. Dry Method

Use the dry construction method at sites where the groundwater level and soil

conditions are suitable to permit construction in relatively dry conditions and

where the sides and bottom of the excavation may be visually inspected before

placing concrete.

i. Unless otherwise accepted by the NDOT, concrete shall be placed in

drilled holes within 24 hours of completing excavation.

ii. All water and loose materials shall be removed from the holes and

reinforcement shall be thoroughly cleaned before concrete is placed.

iii. Concrete shall be placed with a tremie or funnel to prevent segregation.

Use free-fall placement only in dry holes with a maximum 6-foot free-

fall height or NDOT approved height. The concrete shall fall directly to

the pier base without contacting either the pole or hole sidewall. If

concrete placement causes the excavation to cave or slough or if the

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concrete strikes the pole or sidewall, reduce the height of free-fall and

reduce the rate of concrete flow into excavation. If placement cannot be

satisfactorily accomplished by free-fall, use tremie or pumping to place

concrete.

iv. The top 6 feet of concrete shall be rodded or vibrated to provide a dense

mass free of voids. As placed, the concrete shall have a slump between

6 to 8 inches. When scum or laitance accumulates on the top of the

concrete, it shall be removed and replaced with fresh concrete to the

proper elevation.

v. If approved casings are left in place, the void areas between the form

and the excavation walls shall be filled with lean concrete mix. The lean

concrete or grout mix shall be placed and tamped to fill the annular

space.

vi. The volume of each drilled excavation shall be documented and

compared to the concrete volume of each drilled pier. If the concrete

volume placed is less than the calculated (theoretical) volume, the

NDOT shall be notified immediately.

vii. Concrete shall maintain a minimum 6-inch slump for the duration of the

pour.

viii. Self-consolidated concrete may be used meeting NDOT specifications.

In this case, rodding or vibrating per iv above is not necessary.

B. Wet Method

Use the wet construction method or the casing construction method for shafts

that do not meet the above requirements for the dry construction method.

i. Concrete shall not be deposited under water except with NDOT

permission. The proportions for underwater concrete mix shall be

adjusted to provide 7 to 9 inches of slump and the cement factor shall be

increased by one sack per cubic yard.

ii. Underwater concrete shall be placed through a tremie equipped with a

seal at the lower end and a hopper at the upper end. The tremie shall be

watertight and a minimum diameter of 6 times the maximum concrete

aggregate size to allow a free flow of concrete. After the flow of

concrete is started, the lower end of the tremie shall be kept below the

surface of the deposited concrete. The entire mass of concrete shall be

placed as quickly as possible and shall flow into place without shifting

horizontally under the water.

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iii. Fluid within the excavation shall be stable when concrete is deposited,

and shall be maintained at a height necessary to ensure hydrostatic

equilibrium during concrete placement, but not less than 5 ft above the

water table. After placing, the ground water level in the area adjacent to

the drilled shaft shall be kept static (no pumping) until the concrete has

taken its initial set.

iv. Concrete shall maintain a minimum 7-inch slump for the duration of the

pour.

Direct Embedment Installation Record An accurate record of each pier installation and concrete placement shall be

completed that contains, as a minimum, the information listed below. The

Contractor shall submit the installation record to the Company Field Representative

at the end of each day. Submitted records will not become official until the

Company Field Representative agrees with the accuracy and completeness and

signs the document.

The drilled shaft installation record shall contain the following information:

A. Contractor's name

B. Drilled shaft number and location

C. Overall depth of excavation

D. Depth to water

E. Final depth if different from design drawings

F. Note any caving, sloughing of excavation and drilling difficulties

G. Casing insertion, size and length, and whether or not removed

H. Date and time of start and finish excavation

I. Date and time concrete placed

J. Calculated volume of excavation based on diameter of shaft

K. Total actual quantity of concrete or aggregate placed within shaft

excavation

L. Concrete Yield Plot (volume versus shaft depth)

M. Concrete batch plant ticket numbers

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APPENDIX B

Soil Parameters for Nebraska

SPT Blow Counts

SPT has been used for a long time in geotechnical investigation. The method is

conducted by dropping a 140 lb (63.5kgf) hammer from 30 in (762mm) height and hitting the

SPT shoe (also called as a sampler) of a 2 in (50.8 mm) outside diameter and 1.5 in (35 mm)

inside diameter into the ground, and measuring the number of blows required to penetrate the

SPT shoe 12 in (30 mm) into the ground.

The SPT does not utilize any stress and/or strain measurement mechanism, so it does not

provide any direct measurement of strength, deformation, or modulus of soils. However, it is

anticipated that stronger/harder soils may show higher penetration resistance resulting a higher

SPT blow count. Engineers, therefore, have used the SPT blow counts to indirectly obtain the

engineering properties of soils based on empirical charts/correlations for more than two

generations.

In this study, SPT blow counts were practically the only engineering test results

available, therefore, the best engineering judgement was used to obtain required engineering

parameters introduced in Section 3.3.

Correction Factors of SPT Blow Count (N60)

The SPT blow count technique was widely used in the geotechnical area due to its

simplicity and ease of fabrication (USBR, 2020). This simple mechanism and easy to fabricate

feature made the creation of several different versions of SPT testing devices and practices. At

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some point, it was realized that the SPT test results obtained in one area may not be compatible

to SPT test results from another area. Similarly, SPT test results obtained from style A

equipment may not be compatible to SPT test results obtained from style B equipment. As a

solution to secure an interchangeable SPT blow counts, the standardization of SPT has

developed. This involves in equalization of SPT blow count by applying several correction

factors. Following is the summary of correction factors based on Das (2014) for soil samples

from Juneau, Arkansas. This detailed correction method is originally proposed by Skempton

(1986) and Seed, et al. (1985).

Overall form of correction scheme

The N60 can be determined using the following equation:

𝑁60 =𝑁𝜂𝐻𝜂𝐵𝜂𝑆𝜂𝑅

60%

where,

𝑁60 = Standard penetration number for 60% energy ratio

𝑁 = Measured penetration number

𝜂𝐻 = Hammer efficiency (Table B-1)

𝜂𝐵 = Borehole diameter correction (Table B-1)

𝜂𝑆 = Sampler correction (Table B-1)

𝜂𝑅 = Rod length correction (Table B-1)

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Table B-1: Variation of correction factors (retrieved from Das, 2014)

The correction factors in Table B-1 are computed for representative soil conditions as follows:

a) Hammer efficiency:

For United states, safety, rope and pully hammer, ηH = 60% = 0.6

b) Borehole diameter correction:

For diameter (2.4 –4.7 in.), ηB=1

c) Sampler correction:

For no liner, ηS = 1

d) Rod length correction:

For (12 – 20 ft), ηR = 0.85 (average)

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Additional Correction Factors for Cohesionless Soils

The strength and stiffness of cohesionless soils are affected by the confining pressure that

which varies by depth. This is the same with the SPT blow counts. Therefore, additional

correction is applied as follows (Das, 2014, p98):

For sand:

(N1)60 = CNN60

where,

(N1)60 = value of N60 corrected by confining pressure with respect to a standard value

σ’a = Pa [≈ 100 kN/m2 (2000 lb/ft2)].

CN = Correction factor found in Table 3.3.2.

N60 = value of N obtained from the previous relations.

At approximately (3– 15 ft) depth, CN ≈ 1.0 to 1.33 (Normally consolidated coarse sand)

Combined Correction Factor for this Research

From the correction factors introduced previously, the representative correction factors for

the SPT blow counts for this research are as follows.

N60 = 𝑁(0.6x1x1x0.85x (1.0 to 1.33))

0.6

= (0.85 – 1.13) N for cohesionless soils

= 0.85 N for cohesive soils (Correction for the confining pressure not applied.)

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However, it should be noted that some correlations are based on the uncorrected SPT

blow counts and in such cases, uncorrected SPT blow counts should be used.

Internal Friction Angle

The following Figure B-1 shows the typical trend lines of the internal friction angle and

the SPT blow count based on different research (Wolff, 1989; Peck et al. 1974; Hatanaka and

Uchida, 1996; Mayne et al., 2001 and JRA, 1996). From Figure B-1, the reseults from Wolff

(1989), Peck et al. (1974) and JRA (1996) predicted consistent and slightly conservative internal

friction angle.

Figure B-1: Prediction of internal friction angle based on SPT blow count

≈ 1.0 N for both cohesionless soils and cohesive soils because this much difference

in SPT blow count does not make substantial difference in predicted material constant

of soils.

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Therefore, these three methods were used for the prediction of internal friction angles

based on the SPT blow counts. For SPT blow counts ranging between 10 to 25, the graphical

solutions provided an internal friction angle in the range of 27˚ to 35˚.

Cohesion

Cohesion for cohesive soils were also predicted from the SPT blow counts. This study

compared the proposed equations or plots based on corrected SPT blow count and uncorrected

SPT blow count as shown in Tables B-2 and B-3.

Table B-2: Predicted cohesion by corrected SPT blow count

N60-Value < 2 2–4 4–8 8–15 15–30 > 30

Terzaghi and Peck (1976) < 12 12–24 24–48 48–91 91–182 > 182

Hara et al. (1974) < 48 48–79 79–131 131–206 206–340 > 340

Table B-3: Predicted cohesion by uncorrected SPT blow count

N60-Value < 2 2–4 4–8 8–15 15–30 > 30

Terzaghi and Peck (1976) <12.5 12.5–25 25–50 50–100 100–200 > 200

Parcher and Means (1968) <12 12–25 25–50 50–100 100–200 > 200

Tschebotarioff (1973) <15 15–30 30–60 60–120 120–225 > 225

Karol (1960) 12 12–24 24–48 48–96 96–192 > 192

Sowers (1970) (CH) < 24 24–49 49–99 99–186 186–373 > 373

Sowers (1970) (CL) < 14 14–29 29–58 58–109 109–218 > 218

Sowers (1970) (SC–ML) < 7 7–14 14–29 29–54 54–109 > 109

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